image: add support for Netgear encrypted image

image: add support for Netgear encrypted image

Netgear encrypted image is used in various devices including WAX202,
WAX206, and EX6400v3. This image format also requires a dummy squashfs4
image which is added here as well.

References in WAX202 GPL source:
https://www.downloads.netgear.com/files/GPL/WAX202_V1.0.5.1_Source.rar

* openwrt/bootloader/u-boot-mt7621-2018.09-gitb178829-20200526/board/ralink/common/dual_image.c
  Bootloader code that verifies the presence of a squashfs4 image, thus
  a dummy image is added here.

* openwrt/tools/imgencoder/src/gj_enc.c
  Contains code that generates the encrypted image. There is support for
  adding an RSA signature, but it does not look like the signature is
  verified by the stock firmware or bootloader.

* openwrt/tools/imgencoder/src/imagekey.h
  Contains the encryption key and IV. It appears the same key/IV is used
  for other Netgear devices including WAX206 and EX6400v3.

Signed-off-by: Wenli Looi <wlooi@ucalgary.ca>