sysbuild: introduce sysbuild_root for root adjustment

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>