Uses the variable for the default image when adding sysbuild
dependencies so that they can be copied out-of-tree and still work
Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>
rpmsg_service only loads the main application on the target. Switch to
sysbuild which is know to work as expected with the openamp sample.
Signed-off-by: Wilfried Chauveau <wilfried.chauveau@arm.com>