Check for existence of APPLICATION_CONFIG_DIR in sysbuild.
sysbuild.conf is an optional file, meaning that sysbuild will not fail
if APPLICATION_CONFIG_DIR points to a non-existing folder.
A normal Zephyr build already fails when passing an invalid
APPLICATION_CONFIG_DIR but only because it cannot find a required
prj.conf in the folder.
Checking the existence of APPLICATION_CONFIG_DIR in sysbuild and fail
the build when a non-existing folder is passed to sysbuild ensures
sysbuild behaves similar to a normal Zephyr build regarding a
non-existing folder, while still allow sysbuild.conf itself to be
optional.
Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>
6f5b183de7