99-grub-mkconfig: Don't update grubenv generating entries on ppc64le
The grubenv file is updated when grub-mkconfig is executed but on ppc64le
is used on each kernel install to re-generate the grub2.cfg file with the
updated entries. So in this case the grubenv file should not be updated.
Signed-off-by: Javier Martinez Canillas <email@example.com>