sysbuild: Import image devicetrees

Make the image devicetrees available to sysbuild for advanced use cases.

This is done by calling `zephyr_dt_import` on each image target. The DT
target properties are all expected to be prefixed with `DT_`, so there
should be no conflicts.

Thus, the DT contents of a given `<image>` can be retrieved using e.g.:

    dt_nodelabel(<var> NODELABEL <label> TARGET <image>)

as noted in `extensions.cmake`.

Signed-off-by: Grzegorz Swiderski <grzegorz.swiderski@nordicsemi.no>

cmake/modules/extensions.cmake

@@ -3816,6 +3816,10 @@ endfunction()
 #   alias at the beginning of a path interchangeably with the full
 #   path to the aliased node in these functions. The usage comments
 #   will make this clear in each case.
+#
+# - These methods are also available to sysbuild. To retrieve the DT
+#   information of some <image>, after its CMake configuration step,
+#   the dt_* function call must include a "TARGET <image>" argument.
 
 # Usage:
 #   dt_nodelabel(<var> NODELABEL <label> [REQUIRED] [TARGET <target>])

share/sysbuild/cmake/modules/sysbuild_extensions.cmake

@@ -523,6 +523,7 @@ function(ExternalZephyrProject_Cmake)
   endif()
   load_cache(IMAGE ${ZCMAKE_APPLICATION} BINARY_DIR ${BINARY_DIR})
   import_kconfig(CONFIG_ ${BINARY_DIR}/zephyr/.config TARGET ${ZCMAKE_APPLICATION})
+  zephyr_dt_import(EDT_PICKLE_FILE ${BINARY_DIR}/zephyr/edt.pickle TARGET ${ZCMAKE_APPLICATION})
 
   # This custom target informs CMake how the BYPRODUCTS are generated if a target
   # depends directly on the BYPRODUCT instead of depending on the image target.