$MY_REPO/stx/downloads contains links to tarballs and other files
relevant to the build. It is populated by populate_downloads.sh.
On subsequent runs, new links might be added, but old links are
not cleaned up.
Adopt the convention used by generate-centos-repo.sh of moving
the old content under a timestamp backup directory, leaving
an empty directory to re-populate.
Closes-bug: 1908297
Signed-off-by: Scott Little <scott.little@windriver.com>
Change-Id: Ib6c8b782db6c369cb00e8fe8c64be099d7a5d238
1fd6f3f636