sysbuild: split running of CMake from ExternalZephyrProject_add()

Split running CMake from ExternalZephyrProject_add().

This will allow systems to define all Zephyr projects and then at later
stages run the CMake configure stage.

This makes it both cleaner when CMake is invoked as well as prepare for
future work where images could be depending on CMake outcome from other
projects.

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