Switch to the new bmips-multi branch

The old "bcm3384" builds have been deprecated in favor of "bmips-multi".

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
buildsys: Add "distclean" target

This nukes board.dtb (which may have been supplied by the user rather
than build), and zephyr.img (which isn't built by default).

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>