sysbuild: Support BUILD_ONLY images

Add a new parameter to `ExternalZephyrProject_Add()`, which determines
whether a given sysbuild image should only be built and not considered
for flashing and debugging. By adding the following arguments:

   BUILD_ONLY TRUE

the image will be marked as build-only and excluded from `domains.yaml`.

For cases where this setting should be controlled by users or individual
samples, Kconfig can be used:

   ExternalZephyrProject_Add(
     APPLICATION foo
     SOURCE_DIR /path/to/foo
     BUILD_ONLY ${CONFIG_FOO_IS_BUILD_ONLY}
   )

This would be particularly fitting for "general-purpose" images, defined
in-tree or via Zephyr modules (whose inclusion in the multi-image build
should also be Kconfigurable).

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

share/sysbuild/cmake/domains.cmake

@@ -6,6 +6,10 @@ set(domains_yaml "default: ${DEFAULT_IMAGE}")
 set(domains_yaml "${domains_yaml}\nbuild_dir: ${CMAKE_BINARY_DIR}")
 set(domains_yaml "${domains_yaml}\ndomains:")
 foreach(image ${IMAGES})
+  get_target_property(image_is_build_only ${image} BUILD_ONLY)
+  if(image_is_build_only)
+    continue()
+  endif()
   set(domains_yaml "${domains_yaml}\n  - name: ${image}")
   set(domains_yaml "${domains_yaml}\n    build_dir: $<TARGET_PROPERTY:${image},_EP_BINARY_DIR>")
 endforeach()

share/sysbuild/cmake/modules/sysbuild_extensions.cmake

@@ -125,10 +125,13 @@ endfunction()
 #                             For example, -DCONF_FILES=<files> will be passed on to the
 #                             MAIN_APP unmodified.
 #                             BOOTLOADER indicates this app is a bootloader
+# BUILD_ONLY <bool>:          Mark the application as build-only. If <bool> evaluates to
+#                             true, then this application will be excluded from flashing
+#                             and debugging.
 #
 function(ExternalZephyrProject_Add)
   set(app_types MAIN BOOTLOADER)
-  cmake_parse_arguments(ZBUILD "" "APPLICATION;BOARD;BOARD_REVISION;SOURCE_DIR;APP_TYPE" "" ${ARGN})
+  cmake_parse_arguments(ZBUILD "" "APPLICATION;BOARD;BOARD_REVISION;SOURCE_DIR;APP_TYPE;BUILD_ONLY" "" ${ARGN})
 
   if(ZBUILD_UNPARSED_ARGUMENTS)
     message(FATAL_ERROR
@@ -279,6 +282,10 @@ function(ExternalZephyrProject_Add)
       " requires BOARD."
     )
   endif()
+
+  if(DEFINED ZBUILD_BUILD_ONLY)
+    set_target_properties(${ZBUILD_APPLICATION} PROPERTIES BUILD_ONLY ${ZBUILD_BUILD_ONLY})
+  endif()
 endfunction()
 
 # Usage: