[syslinux] Enable debug messages for gpxelinux.0

[Miller, Shao]

Good day David,

Change the Makefile at syslinux/gpxe/Makefile:

  src/bin/undionly.kkpxe: pxelinux.gpxe ../core/pxelinux.0
          $(MAKE) -C src bin/undionly.kkpxe NO_WERROR=1 \
          DEBUG=int13
EMBEDDED_IMAGE=../pxelinux.gpxe,../../core/pxelinux.0

- Shao Miller