gemini: Fix up firmware checksum on DIR-685

gemini: Fix up firmware checksum on DIR-685

Using the same method as the D-Link DAP-2695 A1 we use
the "mtd" tool to augment the firmware checkum in flash
on first boot of a new firmware on the D-Link DIR-685.
We need to augment the Makefile for "mtd" to build in
the special WRGG fixup support for Gemini as well.

This works around the problem of the machine not booting
after factory install unless the sysupgrade is applied
immediately.

Based on commit e3875350f3e4185020b64e0588bba521cd1d6e64
"ar71xx: add support for D-Link DAP-2695 rev. A1"

Cc: Stijn Tintel <stijn@linux-ipv6.be>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>