grubby-bs: Fix changing kernel cmdline params not working on ppc64le
    
    Currently the grubby script re-generates a GRUB config file that contains
    menuentry commands for every /boot/loader/entries/*.conf file snippet.
    
    But this is only needed for ppc64le machines that have a Petitboot version
    that doesn't support parsing BLS snippets. Instead of generating a config
    on any ppc64le machine, restrict this for the case where is really needed.
    
    The /etc/grub.d/10_linux script already takes this into account and does
    not add menuentry commands unless is needed. But the grubby script didn't
    check this, causing changing kernel cmdline params to not work on ppc64le.
    
    Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>