During GRUB terminal input and output setup, we need to make sure that
graphics and serial console work properly for GRUB.

Using both 'console' and 'serial' as GRUB temrinal input and output in
EFI boot mode causes double output, since both 'console' and 'serial'
are writing to serial port. To fix this issue we use 'gfxterm' for EFI
boot mode and make sure that we fallback to serial output only, if
'gfxterm' fails.

For hardware that does not have any graphics option and rely on serial
port, 'gfxterm' would fail with error 'no suitable video mode'. Serial
console will still be enabled in this case.

Signed-off-by: Umer Saleem <usaleem@ixsystems.com>