Transition existing installations to unified GRUB configuration
    
    The previous commits, especially b14117, unified the grub config
    locations across all platforms. In brief, this means that in the
    case of EFI, the config file in the EFI System Partition (ESP)
    is now meant to be a small stub config file that will in turn
    load the main configuration in /boot/grub2, which is used on
    all other platforms as well. For new installations all this is
    done by the Anaconda installer. But existing installations also
    need to be adapted.
    Add a %posttrans script to the grub2-common package that will,
    if a non-unified installation is detected, transition it into
    a unified one. This is done by moving the main grub.cfg file
    from the ESP to /boot/grub2, creating minimal stub on the ESP
    instead. Additionally, the grubenv file is also moved from the
    ESP to /boot/grub2.
    The detection of the non-unified installation is done by
    checking if the grub.cfg on the ESP contains the 'configfile'
    directive. If so, it is assumed the system has a unified
    grub configuration.
    
    Signed-off-by: Christian Kellner <christian@kellner.me>