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>