[efi] Distribute available entropy within stack cookie

Several of the values used to compute a stack cookie (in the absence
of a viable entropy source) will tend to have either all-zeroes or
all-ones in the higher order bits.  Rotate the values in order to
distribute the (minimal) available entropy more evenly.

Suggested-by: Pete Beck <pete.beck@ioactive.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>