tests/util/grub-shell: Enable RNG device to better test stack smashing

tests/util/grub-shell: Enable RNG device to better test stack smashing

In certain firmwares, e.g. OVMF, the RNG protocol is not enabled unless
there is an RNG device. When not enabled, GRUB fails to initialize the
stack guard with random bytes. For testing, this is not a big issue, but
there have been bugs found in the initialization. So turn this on for EFI
platforms to catch any regressions.

Signed-off-by: Glenn Washburn <development@efficientek.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>