Move the autorelabel file to BACKUP_URL instead of OUTPUT_URL.
It's more consistent and help to prepare the "backup in iso" feature
Implement the "backup to iso" feature.
To use that, BACKUP_URL has to use the iso://[subdirs] scheme but
a different OUTPUT_URL has to be defined.
Merge branch 'master' into backupIso
Merge pull request #278 from Arakmar/backupIso

Backup in ISO feature