image: add additional fields to Netgear encrypted image

image: add additional fields to Netgear encrypted image

These fields are used for EAX12 and EX6250v2 series, and perhaps other
devices. Compatibility is preserved with the WAX202 and WAX206.

In addition, adds the related vars to DEVICE_VARS so that the variables
work correctly with multiple devices.

References in GPL source:
https://www.downloads.netgear.com/files/GPL/EAX12_EAX11v2_EAX15v2_GPL_V1.0.3.34_src.tar.gz

* tools/imgencoder/src/gj_enc.c
  Contains code that generates the encrypted image.

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