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 <tomasz.chyrowicz@nordicsemi.no>

share/sysbuild/cmake/modules/sysbuild_extensions.cmake

@@ -283,12 +283,23 @@ function(ExternalZephyrProject_Add)
       )
     endif()
 
+    if(NOT ${ZBUILD_APPLICATION}_DTC_OVERLAY_FILE)
       # 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 ${sysbuild_image_dts_overlay})
-      set(${ZBUILD_APPLICATION}_DTC_OVERLAY_FILE ${sysbuild_image_dts_overlay}
+      set(sysbuild_image_dts_overlay_files ${sysbuild_image_conf_dir}/${ZBUILD_APPLICATION}.overlay)
+
+      # Check for overlay named <ZBUILD_APPLICATION>_<FILE_SUFFIX>.overlay.
+      if(FILE_SUFFIX)
+        list(PREPEND sysbuild_image_dts_overlay_files ${sysbuild_image_conf_dir}/${ZBUILD_APPLICATION}_${FILE_SUFFIX}.overlay)
+      endif()
+
+      foreach(overlay_file ${sysbuild_image_dts_overlay_files})
+        if(EXISTS ${overlay_file})
+          set(${ZBUILD_APPLICATION}_DTC_OVERLAY_FILE ${overlay_file}
             CACHE INTERNAL "devicetree overlay file defined by main application"
           )
+          break()
+        endif()
+      endforeach()
     endif()
   endif()