gemini: Generate padded kernel+rootfs images for DIR-685

gemini: Generate padded kernel+rootfs images for DIR-685

We currently generate a kernel that boots from the harddrive
in the DIR-685. That's not how we usually do things, so
let's augment it to boot from flash and mount the rootfs
using squashfs and JFFS2 like everyone else.

Partition splitting only work when the partitions are
inside of a "partitions" node which is why we have a patch
like this (submitted upstream).

Another patch drops the rootfs arguments and renames the
firmware partition while adding the compatible "wrg"
to it so the WRGG parser will kick in.

Factory image was tested by bravely reflashing the DIR-685
from stock firmware using the web UI and the serial console
boot loader.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>