Some BLS fixes
    
    20-grub-install: Replace, rather than overwrite, the existing kernel (pjones)
      Resolves: rhbz#1642402
    99-grub-mkconfig: Don't update grubenv generating entries on ppc64le
      Related: rhbz#1637875
    blscfg: fallback to default_kernelopts if BLS option field isn't set
      Related: rhbz#1625124
    grub-switch-to-blscfg: copy increment.mod for legacy BIOS and ppc64
      Resolves: rhbz#1652806
    
    Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>