scripts: avoid unnecessary 'west build --force'

Commit 88fb8bac ("scripts: improve west build's board handling")
lets us specify the board with a build.board config or BOARD
environment variable.

However, under some conditions, e.g. if the use has
build.pristine=auto and build.board=some_board, the following fails a
check_force call:

west build samples/hello_world
west build samples/philosophers

The problem is that the check_force wasn't made aware of the other
places a board can come from. Fix that.

Signed-off-by: Marti Bolivar <marti.bolivar@nordicsemi.no>