Snapshot recovery upgrade

[frelon]

The new behavior of deploying the recovery system is prone to errors if an error during the upgrade.

The current method of overwriting the kernel/initrd/bootargs.cfg and symlinks should be switched to a more snapshotty approach deploying the recovery system to a new directory and switching over to use that directory once all files are deployed.

This would also help clean up old files not overwritten during the upgrade.