gemini: fix hard disk boot on D-Link devices

gemini: fix hard disk boot on D-Link devices

Since the D-Link devices boot from hard disk we need to
add the following changes to Gemini:

- Supply a bootarg in the device tree so we can boot from
  the right partition (/dev/sda4 on DNS-313)
- Disable forced command line in config so the kernel picks
  up the right bootargs from the device tree
- Enable EXT4 in the config as this is used for rootfs else
  we get nowhere, we cannot load this as a module because,
  well, it is supposed to be loaded from the root partition
  (chicken and egg problem).
- Enable jbd2 and mbcache (needed by ext4)

Also clean out the premature attempts to dynamically
modify the command line in the Image makefile: we should
pass this with the device tree bootargs instead, it works
much better.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Roman Yeryomin <roman@advem.lv>