Add option --clean to sync_repos.sh

Add option --clean to sync_repos.sh

If this option is passed, sync_repos.sh will clean up all untracked files and
directories that are not listed in .gitigore file. However, if an untracked
diretory is managed by a different Git repository, it's not removed.
Note that the operation is applied for not only in s-* but chromium-efl repo.
The intended client for this option is, but not confined to, the buildbot.

WARNING: Be careful when using this option outside of buildbot context,
         since it can blow up your local changes.

Bug: http://107.108.218.239/bugzilla/show_bug.cgi?id=9584
Reviewed by: Antonio Gomes, Tomasz Olszak, Viatcheslav Ostapenko, mohan reddy

Change-Id: Ic2172925edd691aa3bbfc35fd035c10a0f4f2cea
Signed-off-by: SeungSeop Park <sns.park@samsung.com>