Add support for --overlay and --rsync-{src,dest} to build and osc.

Add support for --overlay and --rsync-{src,dest} to build and osc.
--overlay OVERLAY
              Copy overlay filesystem to buildroot after installing
              all RPMs. This must be a valid directory.
--rsync-src RSYNCSRC
              Copy overlay folder (RSYNCSRC) to a folder (RSYNCDEST)
              inside the buildroot using rsync.
              It will "%define RSYNCDONE 1" for handling %setup in your
              specfile. E.g.:
              %prep
              %if 0%{?RSYNCDONE}
              %setup -n aaa_base -T -D -b 5 -b 7
              %else
              %setup -n aaa_base -b 5 -b 7
              %endif
--rsync-dest RSYNCDEST

Todo: tell rpmbuild not to delete the folders.