[build] Fix genfsimg to build ISO with long filenames

Commit 79c0173 ("[build] Create util/genfsimg for building
filesystem-based images") introduced the new genfsimg, which lacks the
-l option when building ISO files.  This option is required to build
level 2 (long plain) ISO9660 filenames, which are required when using
the .lkrn extensions on older versions of ISOLINUX.

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

src/util/genfsimg

@@ -230,7 +230,7 @@ done
 # since isohybrid will refuse to work without them.
 #
 if [ -n "${ISOIMG}" ] ; then
-    ISOARGS="-J -R"
+    ISOARGS="-J -R -l"
     copy_syslinux_file "isolinux.bin" "${ISODIR}"
     copy_syslinux_file "ldlinux.c32" "${ISODIR}" 2>/dev/null || true
     ISOARGS="${ISOARGS} -no-emul-boot -eltorito-boot isolinux.bin"