Only execute grub2-switch-to-blscfg if GRUB_ENABLE_BLSCFG isn't set
    
    There's no point on executing the script if GRUB_ENABLE_BLSCFG has already
    been set. Currently was checking if an user explicitly set it to false to
    avoid enabling the BLS configuration, but it should also be avoided if was
    already set to true by a previous package update or during installation.
    
    Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>