Add a command to upgrade old repositories to the current version

The command 'upgrade_db' create a dummy repository (to the current version) and
add each images/payloads from the repository to upgrade. Symlinks are used to
avoid copy. Old database is still available in repository as db.bak.

Signed-off-by: Sébastien Luttringer <sebastien.luttringer@smartjog.com>