cmake: improved board handling for revisions

This commit improves board handling for boards in HWMv2.
On a CMake rerun, then BOARD_DIR is passed to `list_boards.py` which
is extended to take such parameter.

This allows to run `list_boards.py` whenever CMake reruns without the
penalty of searching for all board.yml files, as only the board.yml of
the current BOARD_DIR is processed.

This allows `list_boards.py` to be invoked and from there obtain list
of valid revisions and board identifiers for further board validation.

This removes the need for caching additional CMake variables related to
the board identifier and revision and thereby remove the risk of
settings becoming out of sync as only the board provided by user is
needed.

This work further ensure that use-cases described in #50536 is still
supported.

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