Fixes: #69548

Support extending an existing board with new board variants.

This commit introduces the following changes to allow a board to be
extended out-of-tree.

The board yaml schema is extended to support an extend field which
will be used to identify the board to be extended.

A board 'plank' can be extended like this:
> board:
>   extend: plank
>   variants:
>     - name: ext
>       qualifier: soc1

For the rest of the build system this means that there is no longer a
single board directory.
The existing CMake variable BOARD_DIR is kept and reference the
directory which defines the board.
A new CMake variable BOARD_DIRECTORIES provides a list of all
directories which defines board targets for the board.
This means the directory which defines the board as well as all
directories that extends the board.

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