Add support for loading iPXE via a UEFI shim in ISO and USB images.
Since the iPXE shim's default loader filename is currently "ipxe.efi"
for all CPU architectures, at most one architecture within an image
may use a shim. (This limitation should be removed in the next signed
release of the iPXE shim.)
Signed-off-by: Michael Brown <mcb30@ipxe.org>