Some BLS cleanups and fixes
    
    - 20-grub-install: Don't add an id field to generated BLS snippets
    - 99-grub-mkconfig: Disable BLS usage for Xen machines
      Resolves: rhbz#1703700
    - Don't add a class option to menu entries generated for ppc64le
      Resolves: rhbz#1758225
    - 10_linux.in: Also use GRUB_CMDLINE_LINUX_DEFAULT to set kernelopts
    - blscfg: Don't hardcode an env var as fallback for the BLS options field
      Resolves: rhbz#1710483
    
    Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>