ipq40xx: add factory image for EnGenius ENS620EXT

ipq40xx: add factory image for EnGenius ENS620EXT

Extended  mksenaofw to support new "capwap" header structure.
This supports flashing from factory 3.0.0, 3.0.1, 3.1.0 and 3.5.5
firmware.

Note that the factory image format changes for 3.1 and later firmware,
and that the 3.1.0 and 3.5.5 Engenius firmware will refuse the
factory_30.bin file. Similarly, the 3.0.0 and 3.0.1 Engenius firmware
will refuse the factory_35.bin file.

Flashing from the Engenius 3.1.0 firmware with the factory_35.bin
firmware has not been tested, as 3.1.0 firmware (Engenius "middleFW")
is only intended as part of the upgrade path to 3.5.5 firmware.

Modified ipq40xx image Makefile to appropriately invoke mksenaofw
with new parameters to configure the capwap header.

Note that there is currently no method to return to factory firmware,
so this is a one-way street.

Path from factory 3.0.0 and 3.0.1 (EnGenius) software to OpenWrt is
to navigate to 192.168.1.1 on the stock firmware and navigate to the
firmware menu. Then copy the URL you have for that page, something like
http://192.168.1.1/cgi-bin/luci/;stok=12345abcdef/admin/system/flashops
and replace the trailing /admin/system/flashops with just /easyflashops

You should then be presented with a simple "Firmware Upgrade" page.
On that page, BE SURE TO CLEAR the "Keep Settings:" checkbox.

Choose the openwrt-ipq40xx-engenius_ens620ext-squashfs-factory_30.bin,
click "Upgrade" and on the following page select "Proceed".

Path from factory 3.5.5 (EnGenius) software to OpenWrt is simply to
use the stock firmware update menu. Choose the
openwrt-ipq40xx-engenius_ens620ext-squashfs-factory_35.bin and click
"Upload" and "Proceed".

The device should then flash the OpenWrt firmware and reboot. Note
that this resets the device to a default configuration with Wi-Fi
disabled, LAN1/PoE acting as a WAN port (running DHCP client) and LAN2
acting as a LAN port with a DHCP server on 192.168.1.x (AP is at
192.168.1.1)

Signed-off-by: Steve Glennon <s.glennon@cablelabs.com>
Signed-off-by: Christian Lamparter <chunkeey@gmail.com>
[sorry, for unfixing the 80-lines eyesores.]