Make grubby-bls execute grub2-mkconfig on ppc64
    
    When booting an ppc64 machine on bare-metal (PowerNV) the OPAL firmware
    interface is used. The firmware contains the Petitboot boot-loader that
    can be used to parse the BootLoaderSpec (BLS) snippets in a BLS setup.
    
    But machines could have an older version of Petitboot that doesn't have
    BLS support, so on ppc64 machines can't be assumed that just modifying
    the BLS files is enough for those changes to be reflected in the boot
    menu. Instead, grub2-mkconfig is executed so the BLS can be parsed and
    produce a grub config file that can be used by any Petitboot version.
    
    Resolves: rhbz#1636039
    
    Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>