upgrade_oniro: improve backup and restore

Now restore is able to find the latest backup without user
intervention.
Backup will not crash anymore if used without build_directory
set to a correct directory, default is now None.
Rename backup_archive_path to backup_archive_arg to better
represent its meaning.
Add TODOs.

Signed-off-by: Davide Gardenal <davide.gardenal@huawei.com>