Renamed -c option to -r (for "root") and added -c option (for clean).

The old -c option, which specified a 'chroot', has been renamed to -r, since
most other scripts use -r for a different root.  A new -c option is
introduced, which cleans all files from ${chrootdir}/rw before beginning,
thus ensuring a clean chroot.

Signed-off-by: Travis Willard <travis@archlinux.org>
Signed-off-by: Aaron Griffin <aaronmgriffin@gmail.com>