flash: support flashing multiple configurations of a single board

Some boards define multiple configuration which all are maintained under
the same board directory. The flasher was looking for an openocd.cfg
based on the board name, which can't be found for such boards.

Use the variable BOARD_DIR provided by cmake instead of trying to
assemble the board directory location on our own.

Signed-off-by: Anas Nashif <anas.nashif@intel.com>