[efi] Retry calls to GetRNG() as needed

The UEFI specification allows GetRNG() to return EFI_NOT_READY, which
is not a particularly helpful error status since there is nothing that
can sensibly be done except to retry immediately.

Retry failed calls to GetRNG() up to a maximum number of attempts.

Debugged-by: Stoo Davies <sdavies@nvidia.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>