[build] Use SOURCE_DATE_EPOCH for isohybrid MBR ID if it exists

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

src/util/genfsimg

@@ -307,7 +307,11 @@ if [ -n "${ISOIMG}" ] ; then
 	    -publisher "ipxe.org" -sysid "iPXE" -o "${ISOIMG}" \
 	    ${ISOARGS} "${ISODIR}"
     if isohybrid --version >/dev/null 2>&1 ; then
-	isohybrid "${ISOIMG}"
+	ISOHYBRIDARGS=
+	if [ -n "${SOURCE_DATE_EPOCH:-}" ] ; then
+	    ISOHYBRIDARGS="${ISOHYBRIDARGS} --id ${SOURCE_DATE_EPOCH}"
+	fi
+	isohybrid ${ISOHYBRIDARGS} "${ISOIMG}"
     fi
 fi