mediatek: filogic: wax220: support factory image

mediatek: filogic: wax220: support factory image

Enable building a factory image which can be flashed through the OEM
firmware's web interface. It seems that the web interface requires a
minimum file size of 10MiB, otherwise it will not accept the image.

The update image is a regular sysupgrade tarball packed in a Netgear
encrypted image. The Netgear encrypted image is the same as used in
WAX202 or WAX206, including the encryption keys and IV.

This adds a script which creates the rootfs_data volume on first
startup. This is required since the OEM firmware's sysupgrade scripts
do not create such a paritition. Note that any script ordered after
70_initramfs_test will not get executed on initramfs. Hence this new
script 75_rootfs_prepare won't create the rootfs_data volume when
using the recovery initramfs.

Also, this deletes the kernel_backup and rootfs_backup volumes in case
we have to create the rootfs_data volumes. This makes sure that
OpenWrt is the actual backup firmware instead of the stock firmware.

References in WAX220 GPL source:
https://www.downloads.netgear.com/files/GPL/WAX220-V1.0.2.8-gpl-src.tar.gz

* package/base-files/files/lib/upgrade/nand.sh:186
  Creation of rootfs_data is disabled

* Uboot-upstream/board/mediatek/common/ubi_helper.c
  Automatic creation of UBI backup volumes

Signed-off-by: Stefan Agner <stefan@agner.ch>