cmake: support relocating of a board folder

Fixes: #49116

During development of out-of-tree boards and applications it is not
uncommon to refactor / restructure code.

To allow developers more freedom, let's check that board's defconfig
still exists during a CMake re-run.
If the defconfig no longer exists, either because it's been moved or
deleted, then warn the user and set BOARD_DIR to NOTFOUND.

The NOTFOUND will request CMake to search for the new location in all
board roots. If the board has not been found, as example it's deleted,
then the existing error is printed later.

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