ar71xx: create new subtarget nand-yaffs2

This target will be used for Mikrotik devices using YAFFS2 flash.
The subtarget nand will support NAND devices on UBI/UBIFS.
ar71xx: altered subtarget nand for UBI/UBIFS

Altered the subtarget nand, use UBI/UBIFS instead of YAFFS2.
Added the first board WNDR4000 to the subtarget to generate
an initramfs image with NAND/UBI/UBIFS support.
ar71xx: Added ubinize.cfg and UBI flags to image Makefile

In order to generate UBI root filesytems the ubinize.cfg needs
to be in place. The Makefile provides working default parameters
for WNDR4300 used by the UBI utilities.
ar71xx: factory image generation for WNDR4300

Add factory image generation using UBI/UBIFS. The rootfile is
mounted rw, no overlay support yet.

Just in front of the rootfs (still on the kernel partition),
a fake header needs to be in place to make stock U-Boot happy.
This fake header tells the system rootfs is 4 bytes long. The
first for bytes of the rootfs is an UBI magic (ASCII UBI#).
ar71xx: Empty whole rootfs partition on WNDR4300

UBIFS needs the whole partition to be empty upon first mounting.
Since the bootloader only flashes the used amount of the UBI
image, this patch padds the image with 0xff.
ar71xx: Add new target template ubi

Since we need the UBIFS/UBI image generated by include/image.mk,
we need to make sure NetgearNAND images get built after mkfs/ubifs
was called.
ar71xx: Move WNDR4300 from the generic target to NAND-subtarget

By moving the WNDR4300 to the NAND-subtarget, the correct image will
be created immediately.
ar71xx: Move UBI and UBIFS options to WNDR4300 profile.

The generic subtarget has got nothing to do with UBI or UBIFS (yet),
so the definitions were moved into the profile for the WNDR4300.
This also ensures that possible future devices do not have a non-
working preset by accident.
ar71xx: Let NetgearNAND/initramfs use Netgear/initramfs

So that the initramfs images get created at least semi-
correctly for NetgearNAND target devices.
ar71xx: Move the final image preparations to their own subroutine

By moving the final preparations to their own routine and calling
them in this way, it is ensured that we only try to read the
root.ubi file after it has been created. Furthermore, we can add
a custom routine for the ubifs image, so we can use that to
eventually create a working sysupgrade image.