configs: update cubieboard2 defconfig

configs: update cubieboard2 defconfig

This patch updates the Cubieboard2 board defconfig to use newer u-boot
and linux versions, along with a few other improvements.
The other improvements were mostly backported from the recent Orange Pi
defconfig inclusion and are related to image generation.
linux-sunxi.org was used as a reference for both linux and u-boot
defconfig names.

In detail:
- Update linux to vanilla upstream -- currently 4.3.3 is the most
up-to-date. Although it lacks some features present only in the
linux-sunxi 3.4 branch, upstream support appears to be relatively mature
and is already being deployed by default by distributions such as Arch
Linux ARM.
- Update u-boot to vanilla upstream 2015.10. According to
linux-sunxi.org, upstream fully supports major functions (except NAND)
since 2015.07.
- Change image format to zImage and drop custom image generation script
in favour of genimage, as pointed out by reviewers in the Orange Pi
defconfig submission.

This was tested in a Cubieboard2 board with boot from mmc and it boots
fine to login prompt.

This has not been tested in a Cubieboard (A10) as I don't own any; the
version bumps probably apply to it although there are some differences
(lack of VFPv4 support, Cortex-A8 instead of Cortex-A7), so I have kept
the current Cubieboard defconfig and its scripts. This one has been only
build-tested.

Signed-off-by: Erico Nunes <nunes.erico@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>