repocache: clean out stale submodules, too

repocache: clean out stale submodules, too

Use "double force" when doing git-clean, in order to remove untracked
submodule directories, too. From git-clean manpage:
    -d  Remove untracked directories in addition to untracked files. If
        an untracked directory is managed by a different Git repository,
        it is not removed by default. Use -f option twice if you really
        want to remove such a directory.

Change-Id: I95235d492bdb943c26316843b7d823b7d07b427b
Signed-off-by: Markus Lehtonen <markus.lehtonen@linux.intel.com>