grub2: fix some quirks and div by zero

grub2: fix some quirks and div by zero

Rather than erroring out on a single attempt while
terminating EFI services, make a few retries because
such quirks are found in a few implementations.
Also fix a div by zero issue in the same framework
which causes an infinite reboot on the target.
Both patches included here are backports.

Signed-off-by: Awais Belal <awais_belal@mentor.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>