Enable builds for armv6 target

[rahul-thakoor]

We currently build for

        aarch64-unknown-linux-gnu,
        armv7-unknown-linux-gnueabihf,
        x86_64-unknown-linux-gnu,
        i686-unknown-linux-gnu

I tried to enable armv6 builds, using target arm-unknown-linux-gnueabihf

Cross compiling libdbus fails. As per instructions here I added

[target.arm-unknown-linux-gnueabihf.env]
passthrough = ["RUSTFLAGS=-L /usr/arm-linux-gnueabihf/lib/ -L /usr/lib/arm-linux-gnueabihf/", "PKG_CONFIG_PATH=/usr/lib/arm-linux-gnueabihf/pkgconfig/","PKG_CONFIG_ALLOW_CROSS=true"]

but Flowzone build fails in the Publish step with :

Compiling wifi-connect v4.11.38 (/project)
     Running `rustc --crate-name wifi_connect src/main.rs --error-format=json --json=diagnostic-rendered-ansi,artifacts,future-incompat --crate-type bin --emit=dep-info,link -C opt-level=3 -C lto -C metadata=2b3a8d67df011610 -C extra-filename=-2b3a8d67df011610 --out-dir /target/arm-unknown-linux-gnueabihf/release/deps --target arm-unknown-linux-gnueabihf -C linker=arm-unknown-linux-gnueabihf-gcc -L dependency=/target/arm-unknown-linux-gnueabihf/release/deps -L dependency=/target/release/deps --extern clap=/target/arm-unknown-linux-gnueabihf/release/deps/libclap-1fc5a8f8b647bd63.rlib --extern env_logger=/target/arm-unknown-linux-gnueabihf/release/deps/libenv_logger-8ffd08e5a27961c7.rlib --extern error_chain=/target/arm-unknown-linux-gnueabihf/release/deps/liberror_chain-756e7fcd699e1ec1.rlib --extern iron=/target/arm-unknown-linux-gnueabihf/release/deps/libiron-f44288b333742354.rlib --extern iron_cors=/target/arm-unknown-linux-gnueabihf/release/deps/libiron_cors-037231c07bde4d0a.rlib --extern log=/target/arm-unknown-linux-gnueabihf/release/deps/liblog-5ca2f0517caaf135.rlib --extern mount=/target/arm-unknown-linux-gnueabihf/release/deps/libmount-a113c6e0a1459beb.rlib --extern network_manager=/target/arm-unknown-linux-gnueabihf/release/deps/libnetwork_manager-0bded0b8b9176299.rlib --extern nix=/target/arm-unknown-linux-gnueabihf/release/deps/libnix-5bec8111479cc868.rlib --extern params=/target/arm-unknown-linux-gnueabihf/release/deps/libparams-4e183d770ec1db4a.rlib --extern persistent=/target/arm-unknown-linux-gnueabihf/release/deps/libpersistent-fb28a16d6c776729.rlib --extern router=/target/arm-unknown-linux-gnueabihf/release/deps/librouter-7cbae9fe3b9d325d.rlib --extern serde=/target/arm-unknown-linux-gnueabihf/release/deps/libserde-988a08d2dde734ac.rlib --extern serde_derive=/target/release/deps/libserde_derive-3911f2c984408e78.so --extern serde_json=/target/arm-unknown-linux-gnueabihf/release/deps/libserde_json-fef308203ef5d039.rlib --extern staticfile=/target/arm-unknown-linux-gnueabihf/release/deps/libstaticfile-98284c1a32ce0904.rlib -L /usr/arm-linux-gnueabihf/lib/ -L /usr/lib/arm-linux-gnueabihf/ -L native=/usr/lib/arm-linux-gnueabihf`
error: linking with `arm-unknown-linux-gnueabihf-gcc` failed: exit status: 1
  |
  = note: LC_ALL="C" PATH="/rust/lib/rustlib/x86_64-unknown-linux-gnu/bin:/x-tools/arm-unknown-linux-gnueabihf/bin/:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/rust/bin" VSLANG="1033" "arm-unknown-linux-gnueabihf-gcc" "/tmp/rustc2PNj5B/symbols.o" "/target/arm-unknown-linux-gnueabihf/release/deps/wifi_connect-2b3a8d67df011610.wifi_connect.ce89b739002be5e2-cgu.11.rcgu.o" "-Wl,--as-needed" "-L" "/target/arm-unknown-linux-gnueabihf/release/deps" "-L" "/target/release/deps" "-L" "/usr/arm-linux-gnueabihf/lib/" "-L" "/usr/lib/arm-linux-gnueabihf/" "-L" "/usr/lib/arm-linux-gnueabihf" "-L" "/rust/lib/rustlib/arm-unknown-linux-gnueabihf/lib" "-Wl,-Bstatic" "/rust/lib/rustlib/arm-unknown-linux-gnueabihf/lib/libcompiler_builtins-76bc871ffb0f557c.rlib" "-Wl,-Bdynamic" "-ldbus-1" "-lgcc_s" "-lutil" "-lrt" "-lpthread" "-lm" "-ldl" "-lc" "-Wl,--eh-frame-hdr" "-Wl,-z,noexecstack" "-L" "/rust/lib/rustlib/arm-unknown-linux-gnueabihf/lib" "-o" "/target/arm-unknown-linux-gnueabihf/release/deps/wifi_connect-2b3a8d67df011610" "-Wl,--gc-sections" "-pie" "-Wl,-z,relro,-z,now" "-Wl,-O1" "-nodefaultlibs"
  = note: /x-tools/arm-unknown-linux-gnueabihf/lib/gcc/arm-unknown-linux-gnueabihf/8.3.0/../../../../arm-unknown-linux-gnueabihf/bin/ld: warning: libsystemd.so.0, needed by /usr/lib/arm-linux-gnueabihf//libdbus-1.so, not found (try using -rpath or -rpath-link)
          /x-tools/arm-unknown-linux-gnueabihf/lib/gcc/arm-unknown-linux-gnueabihf/8.3.0/../../../../arm-unknown-linux-gnueabihf/bin/ld: /usr/lib/arm-linux-gnueabihf//libdbus-1.so: undefined reference to `sd_listen_fds@LIBSYSTEMD_209'
          /x-tools/arm-unknown-linux-gnueabihf/lib/gcc/arm-unknown-linux-gnueabihf/8.3.0/../../../../arm-unknown-linux-gnueabihf/bin/ld: /usr/lib/arm-linux-gnueabihf//libdbus-1.so: undefined reference to `sd_is_socket@LIBSYSTEMD_209'
          collect2: error: ld returned 1 exit status
          
  = note: some `extern` functions couldn't be found; some native libraries may need to be installed or have their path specified
  = note: use the `-l` flag to specify native libraries to link
  = note: use the `cargo:rustc-link-lib` directive to specify the native libraries to link with Cargo (see https://doc.rust-lang.org/cargo/reference/build-scripts.html#cargorustc-link-libkindname)

error: could not compile `wifi-connect` (bin "wifi-connect") due to 1 previous error

Caused by:
  process didn't exit successfully: `rustc --crate-name wifi_connect src/main.rs --error-format=json --json=diagnostic-rendered-ansi,artifacts,future-incompat --crate-type bin --emit=dep-info,link -C opt-level=3 -C lto -C metadata=2b3a8d67df011610 -C extra-filename=-2b3a8d67df011610 --out-dir /target/arm-unknown-linux-gnueabihf/release/deps --target arm-unknown-linux-gnueabihf -C linker=arm-unknown-linux-gnueabihf-gcc -L dependency=/target/arm-unknown-linux-gnueabihf/release/deps -L dependency=/target/release/deps --extern clap=/target/arm-unknown-linux-gnueabihf/release/deps/libclap-1fc5a8f8b647bd63.rlib --extern env_logger=/target/arm-unknown-linux-gnueabihf/release/deps/libenv_logger-8ffd08e5a27961c7.rlib --extern error_chain=/target/arm-unknown-linux-gnueabihf/release/deps/liberror_chain-756e7fcd699e1ec1.rlib --extern iron=/target/arm-unknown-linux-gnueabihf/release/deps/libiron-f44288b333742354.rlib --extern iron_cors=/target/arm-unknown-linux-gnueabihf/release/deps/libiron_cors-037231c07bde4d0a.rlib --extern log=/target/arm-unknown-linux-gnueabihf/release/deps/liblog-5ca2f0517caaf135.rlib --extern mount=/target/arm-unknown-linux-gnueabihf/release/deps/libmount-a113c6e0a1459beb.rlib --extern network_manager=/target/arm-unknown-linux-gnueabihf/release/deps/libnetwork_manager-0bded0b8b9176299.rlib --extern nix=/target/arm-unknown-linux-gnueabihf/release/deps/libnix-5bec8111479cc868.rlib --extern params=/target/arm-unknown-linux-gnueabihf/release/deps/libparams-4e183d770ec1db4a.rlib --extern persistent=/target/arm-unknown-linux-gnueabihf/release/deps/libpersistent-fb28a16d6c776729.rlib --extern router=/target/arm-unknown-linux-gnueabihf/release/deps/librouter-7cbae9fe3b9d325d.rlib --extern serde=/target/arm-unknown-linux-gnueabihf/release/deps/libserde-988a08d2dde734ac.rlib --extern serde_derive=/target/release/deps/libserde_derive-3911f2c984408e78.so --extern serde_json=/target/arm-unknown-linux-gnueabihf/release/deps/libserde_json-fef308203ef5d039.rlib --extern staticfile=/target/arm-unknown-linux-gnueabihf/release/deps/libstaticfile-98284c1a32ce0904.rlib -L /usr/arm-linux-gnueabihf/lib/ -L /usr/lib/arm-linux-gnueabihf/ -L native=/usr/lib/arm-linux-gnueabihf` (exit status: 1)