Add current minimal-uefi for aarch64, stop allowing "current"
    
    "current" is just a synonym for "-1" but made the code more
    complex. Let's ditch it and just note in a comment that -1 is
    CURRREL and -2 is PREVREL. We need minimal-uefi for aarch64 as
    install_blivet_btrfs_preserve_home_uefi uses it.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>