[pxeprefix] Ensure that UNDI IRQ is disabled before starting iPXE

[pxeprefix] Ensure that UNDI IRQ is disabled before starting iPXE

When using the undionly.kkpxe binary (which is never recommended), the
UNDI interrupt may still be enabled when iPXE starts up.  If the PXE
base code interrupt handler is not well-behaved, this can result in
undefined behaviour when interrupts are first enabled (e.g. for
entropy gathering, or for allowing the timer tick to occur).

Fix by detecting and disabling the UNDI interrupt during the prefix
code.

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