Implement actual build dependency between target and linker script

[kilograham]

We have the delightful

    # LINKER script will be PICO_TARGET_LINKER_SCRIPT if set on target, or ${CMAKE_CURRENT_LIST_DIR}/memmap_foo.ld
    # if PICO_TARGET_BINARY_TYPE is set to foo on the target, otherwise ${CMAKE_CURRENT_LIST_DIR}/memmap_${PICO_DEFAULT_BINARY_TYPE).ld
    target_link_options(pico_standard_link INTERFACE
            "LINKER:--script=$<IF:$<BOOL:$<TARGET_PROPERTY:PICO_TARGET_LINKER_SCRIPT>>,$<TARGET_PROPERTY:PICO_TARGET_LINKER_SCRIPT>,${CMAKE_CURRENT_LIST_DIR}/memmap_$<IF:$<STREQUAL:$<TARGET_PROPERTY:PICO_TARGET_BINARY_TYPE>,>,${PICO_DEFAULT_BINARY_TYPE},$<TARGET_PROPERTY:PICO_TARGET_BINARY_TYPE>>.ld>"
    )

But this seems to confuse the (i assume otherwise) automatic dependency added by CMake

Having been bitten by this for the nth time i took a quick look, and pass the full target property to pico_add_link_depend seems to actually do the trick

pico_add_link_depend(pico_standard_link "<IF:$<BOOL:$<TARGET_PROPERTY:PICO_TARGET_LINKER_SCRIPT>>,$<TARGET_PROPERTY:PICO_TARGET_LINKER_SCRIPT>,${CMAKE_CURRENT_LIST_DIR}/memmap_$<IF:$<STREQUAL:$<TARGET_PROPERTY:PICO_TARGET_BINARY_TYPE>,>,${PICO_DEFAULT_BINARY_TYPE},$<TARGET_PROPERTY:PICO_TARGET_BINARY_TYPE>>.ld>")

seems to do the trick, but this probably needs a bit more thorough testing, and also will want to avoid double-jeopardy with pico_set_linker_script (i.e. dont add the dependency twice, but also to check that it indeed works - and doesn't affect multiple binaries in the same project)