Entropy gathering via timer ticks is slow under UEFI (of the order of
20-30 seconds on some machines).  Use the EFI_RNG_PROTOCOL if
available, to speed up the process of entropy gathering.

Note that some implementations (including EDK2) will fail if we
request fewer than 32 random bytes at a time, and that the RNG
protocol provides no guarantees about the amount of entropy provided
by a call to GetRNG().  We take the (hopefully pessimistic) view that
a 32-byte block returned by GetRNG() will contain at least the 1.3
bits of entropy claimed by min_entropy_per_sample().

Signed-off-by: Michael Brown <mcb30@ipxe.org>