This allows:
$ make BOARD_DIR=path/to/board
to infer BOARD=board, rather than the previous behavior that required
additionally setting BOARD explicitly.
Also makes the same change for VARIANT_DIR -> VARIANT on Unix.
This work was funded through GitHub Sponsors.
Signed-off-by: Jim Mussared <jim.mussared@gmail.com>
Signed-off-by: Damien George <damien@micropython.org>