Draft: Fix rollback-blacklist test on EFI systems
The rollback-blacklist test is currently failing on amd64 using GRUB EFI systems.
This MR fixes the case where the alternative boot conf has been removed, which happens after a rollback.
It also fixes the checking on the update flag after a successful update has been performed. However, this is an ongoing discussion as the u-boot case is not matching this behavior.