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 partition on first
startup. This is required since the OEM firmware's sysupgrade scripts
do not create such a paritition.

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 <[email protected]>

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.

Signed-off-by: Stefan Agner <[email protected]>

mediatek: filogic: wax220: support factory image

Enable building a factory image which can be flashed through the OEM
firmware's web interface.

The Netgear encrypted image seems to be the same as used in WAX202 or
WAX206, including the encryption keys and IV.

Signed-off-by: Stefan Agner <[email protected]>