Backported AUM from v2022dev1 -> v2021pre

Original: !59 (merged)

During the major upgrade of the Apertis OS with static delta we need to use the commit containing current branch name in metadata. Such commit would be accepted by AUM and libostree, so the updated OS will be booted with new major version.

Due the nature of offline upgrade the branch name didn't changed to the new name and AUM should to care about refs update.

In case if the system has been booted well we mark the update as successful. Additional step has been added for marking the update as complete -- read branches names from the commit metadata and set repository to the newest version of Apertis if needed. After this step the ostree repository will be configured for new branch (release) usage.

Since all checks are done on the recent Apertis version during the boot attempt, there is no need to change anything on the old system.