grubby-bls: don't update grubenv when generating grub.cfg for ppc64le
    
    Since PowerNV machines can have a Petitboot versions that still don't have
    BLS support, grubby re-generates the grub.cfg file on all ppc64le machines.
    
    But this has the side effect that the grubenv file is updated, which will
    overwrite any value that was set by grubby itself. To prevent that run the
    grub2-mkconfig with the --no-grubenv-update option.
    
    Related: rhbz#1726514
    
    Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>