[build] Include EFI system partition table entry in isohybrid images

Add the "--uefi" option when invoking isohybrid on an EFI-bootable
image, to create a partition mapping to the EFI system partition
embedded within the ISO image.

This allows the resulting isohybrid image to be booted on UEFI systems
that will not recognise an El Torito boot catalog on a non-CDROM
device.

Originally-fixed-by: Christian Hesse <mail@eworm.de>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/util/genfsimg

@@ -308,6 +308,9 @@ if [ -n "${ISOIMG}" ] ; then
 	    ${ISOARGS} "${ISODIR}"
     if isohybrid --version >/dev/null 2>&1 ; then
 	ISOHYBRIDARGS=
+	if [ -n "${EFI}" ] ; then
+	    ISOHYBRIDARGS="${ISOHYBRIDARGS} --uefi"
+	fi
 	if [ -n "${SOURCE_DATE_EPOCH:-}" ] ; then
 	    ISOHYBRIDARGS="${ISOHYBRIDARGS} --id ${SOURCE_DATE_EPOCH}"
 	fi