sysbuild: support application CMakePresets.json files with sysbuild

This commit provides a CMakePresets.json which includes the sample's
CMakePresets.json file.

`west build` is extended to set `APP_DIR` in environment when sysbuild
is used. This allows sysbuild's CMakePresets.json to include the
sample's presets file.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>