cmake: sysbuild: Add support for recurive target cmake inclusion

Adds support for all sysbuild targets to have a sysbuild.cmake file
included and processed as part of sysbuild instead of just the main
application.

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>
Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>