sysbuild: Allow to use file suffix DTS overlays

Allow to add an image-specific DTS overlay, that is applied based on the
FILE_SUFFIX.
Up to this point, it was possible to specify such Kconfig files
(sysbuild/<image>_<file_suffix>.conf),
but it did not work with dts overlays
(sysbuild/<image>_<file_suffix>.overlay).
This commit aligns the logic of finding configuration files for both
scenarios mentioned above.

Ref: NCSDK-NONE

Signed-off-by: Tomasz Chyrowicz <[email protected]>

Author: tomchy

share/sysbuild/cmake/modules/sysbuild_extensions.cmake

@@ -283,6 +283,14 @@ function(ExternalZephyrProject_Add)
       )
     endif()
 
+    # Check for overlay named <ZBUILD_APPLICATION>_<FILE_SUFFIX>.overlay.
+    set(sysbuild_image_dts_overlay ${sysbuild_image_conf_dir}/${ZBUILD_APPLICATION}_${FILE_SUFFIX}.overlay)
+    if (NOT ${ZBUILD_APPLICATION}_DTC_OVERLAY_FILE AND EXISTS ${sysbuild_image_dts_overlay})
+      set(${ZBUILD_APPLICATION}_DTC_OVERLAY_FILE ${sysbuild_image_dts_overlay}
+          CACHE INTERNAL "devicetree overlay file defined by main application"
+      )
+    endif()
+
     # Check for overlay named <ZBUILD_APPLICATION>.overlay.
     set(sysbuild_image_dts_overlay ${sysbuild_image_conf_dir}/${ZBUILD_APPLICATION}.overlay)
     if (NOT ${ZBUILD_APPLICATION}_DTC_OVERLAY_FILE AND EXISTS ${sysbuild_image_dts_overlay})