Zephyr: Boot directly from flash (XIP)

[mateusz-holenko]

Some platforms (e.g., icebreaker) boot directly from flash (XIP), what is not currently supported by the Zephyr binaries built by litex-buildenv.

Once LiteX spi flash dirver is merged to Zephyr, scripts/build-zephyr.sh script should be extended to allow building images capable of XIP.

Once it's done, filtering in scripts/test-renode.sh should be removed.