grub-reboot: Warn when "for the next boot only" promise cannot be kept

grub-reboot: Warn when "for the next boot only" promise cannot be kept

The "for the next boot only" property of grub-reboot is dependent upon
GRUB being able to clear the next_entry variable in the environment
block. However, GRUB cannot write to devices using the diskfilter
and lvm abstractions.

Ref: https://lists.gnu.org/archive/html/grub-devel/2009-12/msg00276.html
Ref: https://bugs.launchpad.net/bugs/788298

Signed-off-by: dann frazier <dann.frazier@canonical.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>