gemini: Generate harddisk image for DNS-313

gemini: Generate harddisk image for DNS-313

This makes OpenWrt build an Ext2+Ext4 partitioned image
for the D-Link DNS-313 with two blank partitions, a boot
partition on /dev/sda3 and a rootfs partition on
/dev/sda4.

This uses the methods already used in apm821xx including
the tricks to convert the generated Ext2 filesystem
partition to version 1 so that the firmware bootloader
will properly recognize it.

We patch a bit around the build files to make sure we
get the rootfs size set and that genext2fs is properly
built.

Tested on the D-Link DNS-313.

Signed-off-by: Christian Lamparter <chunkeey@gmail.com>
[Fixing and tricksing]
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>