cmake: include pre_dt_board.cmake from all board directories

After zephyrproject-rtos#72857 the concept
of board directory was expanded from a single path (BOARD_DIR) to a list
of paths (BOARD_DIRECTORIES) which is taken into account in several places
except when including `pre_dt_board.cmake` files. This patch moves the
inclusion of `pre_dt_board.cmake` files into `dts.cmake` AFTER
`boards.cmake` is included (so we can access `BOARD_DIRECTORIES`) but
BEFORE `pre_dt.cmake`, which preserves the original inclusion order
relative to pre DTC operations.

The original motivation of this patch was to address the practicality of
defining board extensions that can re-use base board definitions.

In particular, one of the drawbacks of
zephyrproject-rtos#72857 is that board
extensions are not really extending board definitions just the qualifier.
There is still the need to create a DTS for each new board extension which
is often a copy-and-paste of the base board DTS eventually with a few
alterations. The problem is that except for toy examples, board DTS files
are rarely self-contained and instead are composed of multiple dtsi
includes using relative paths which then forces board extension files to
either use hard coded paths to the base board directory
OR recursively copy all included dtsi files from the base board directory
into the board extension directory.

With this patch a board extension directory can define custom
configuration steps like regular base boards can and then adjust CPP and
DTC include search paths dynamically, for example with:
```
list(APPEND DTS_EXTRA_CPPFLAGS "-I${BOARD_DIR}")
list(APPEND EXTRA_DTC_FLAGS "-i${BOARD_DIR}")
```

Signed-off-by: Nicolas Lebedenco <[email protected]>

Author: nlebedenco

cmake/modules/dts.cmake

@@ -5,6 +5,12 @@ include_guard(GLOBAL)
 include(extensions)
 include(python)
 include(boards)
+
+# Include board specific device-tree flags before parsing.
+foreach(dir IN LISTS BOARD_DIRECTORIES)
+  include("${dir}/pre_dt_board.cmake" OPTIONAL)
+endforeach()
+
 include(pre_dt)
 find_package(HostTools)
 find_package(Dtc 1.4.6)

cmake/modules/zephyr_default.cmake

@@ -101,10 +101,6 @@ list(APPEND zephyr_cmake_modules hwm_v2)
 list(APPEND zephyr_cmake_modules configuration_files)
 list(APPEND zephyr_cmake_modules generated_file_directories)
 
-# Include board specific device-tree flags before parsing.
-set(pre_dt_board "\${BOARD_DIR}/pre_dt_board.cmake" OPTIONAL)
-list(APPEND zephyr_cmake_modules "\${pre_dt_board}")
-
 # DTS should be close to kconfig because CONFIG_ variables from
 # kconfig and dts should be available at the same time.
 list(APPEND zephyr_cmake_modules dts)