Fixes: #73066
Introduce sysbuild_root CMake module similar to the Zephyr root CMake
module. The sysbuild_root CMake module works similar to existing root
module, but with the difference that root paths are adjusted relative to
APP_DIR which is the main image, instead of relative to sysbuild.
Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>
