Cleanup .boards.depend when using an objtree

.boards.depend was created in the source tree even when calling make
with O=objtree, and distclean O=objtree wouldn't clean it.  Create
.boards.depend in objtree instead as to clean it up properly.

Reported-by: Loc Minier <loic.minier@linaro.org>
Signed-off-by: Wolfgang Denk <wd@denx.de>