cmake: enhanced board entry file handling

[tejlmand]

With a single board now covering what used to be several boards, and with the ability to omit SoC when building for a single SoC board, then <board>_defconfig and <board>.dts lookup is improved.

A single SoC board may prefer to keep its defconfig entry point as <board>_defconfig instead of <board>_<soc>_defconfig.

Also, a multi-SoC board / multi-core SoC board, which used to be implemented as n-boards may wish to have common _defconfig settings in a common <board>_defconfig file, and the SoC / cpuset specifics in <board>_<soc>_defconfig / <board>_<soc>_<core>_defconfig.

Such defconfig support allows also to place build variant specifics in its own <board>_<soc>_<variant>_defconfig file.

This commit allows multiple _defconfigs for a board and its identifiers.

Similar is implemented for a board's dts file.
If a <board>_<soc>_<core>.dts file is not found, the build system will instead use <board>_<soc>.dts, and finally fallback to <board>.dts.

This allows a board to have a shared dts file for all board identifiers which are identical while still support specific dts where required.

A dts file is a devicetree starting point and thus two dts files cannot be used in together. For such cases, an ordinary board overlay file must be used.

[fabiobaltieri]

@tejlmand you may want to look at LLVM being sad, some tests are timing out, unclear to me why

[57300]

Based on this description, I tried:

zephyr_build_string(build_string BOARD alpha BOARD_REVISION 1.0.0 BOARD_IDENTIFIER /soc/bar BUILD foo MERGE)

# I expected:  alpha_soc_bar_1_0_0_foo;alpha_soc_bar_1_0_0;alpha_soc_bar;alpha_soc;alpha
# but I got:   alpha_soc_bar_1_0_0_foo;alpha_soc_bar;alpha

It works if I streamline the implementation like this:

  string(REPLACE "/" ";" build_str_segments "${BUILD_STR_BOARD}${BUILD_STR_BOARD_IDENTIFIER}")
  if(DEFINED BUILD_STR_BOARD_REVISION)
    string(REPLACE "." "_" revision_string ${BUILD_STR_BOARD_REVISION})
    list(APPEND build_str_segments ${revision_string})
  endif()
  list(APPEND build_str_segments ${BUILD_STR_BUILD})

  while(NOT build_str_segments STREQUAL "")
    list(JOIN build_str_segments "_" variant_string)
    list(APPEND ${outvar} "${variant_string}")
    list(POP_BACK build_str_segments)
    if(NOT BUILD_STR_MERGE)
      break()
    endif()
  endwhile()

[tejlmand]

Nice catch 👍
And thanks for the proposal, using JOIN sure is cleaner.

[tejlmand]

fixed, resorted to use string(JOIN ...) as that allows for BUILD_STR_BUILD at end, so that all build strings are appended with that info when passed as argument.

That conforms to what we do today with build string, and it was an oversight in first round that it was only appended to first string.

Function description updated as well.

[tejlmand]

compliance failure is related to out-of-date MAINTAINERS file for soc and board entries.
MAINTAINERS file is planned to be updated in a single commit when porting is done.