aarch64 kernel arg fixup: fix for bootupd
    
    We now have some images using bootupd, at least Fedora IoT images
    from F40 onwards do. These don't have a /etc/default/grub and
    don't really intend you to run grub2-mkconfig. As advised at
    https://github.com/ostreedev/ostree/pull/3150#issuecomment-1998768240
    let's just add our required arguments directly in the BLS snippet
    files.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>