In 'make clean', whether using the old layout or USE_NEW_LAYOUT,
we need to blow away a directory such as $AVATAR_ROOT/FreeNAS-9.2.2-ALPHA-20140327/
which is created by the release script.

Later on, we should change the release script so that it puts
this generated directory under
$AVATAR_ROOT/../obj/release/FreeNAS-9.2.2-ALPHA-20140327/

so that generated directories don't get put under the checked out source
directory.