lantiq: DGN3500 (all known variants) factory and sysupgrade image building support

This patch adds factory image building for the DGN3500, all variants,
and fixes sysupgrade images to make them play nice with the sercomm
secondary boot loader.

The factory images can be used directly in the update dialog in the
interface of the stock firmware and via the special Sercomm bootmode
and a special windows flashing utility (allegedly present in the CD
that came with the device -- but it's also compatible with the NSLU2
Upgrade_207_XP utility.) The special bootmode can be activated by
turning the device on while holding the reset button pressed, then
releasing it when the power led starts blinking red and green. Please
notice that if using the 207 utility, it will always report that the
flashing failed even though it completed successfully. Just power
cycle the router manually after the utility reports the failure and
OpenWRT will boot. This same utility (despite reporting failure in
this case too) can revert a DGN3500 (any variant) to the appropriate
stock Netgear firmware.

This patch is a heavily modified version of a package I found on the
OpenWRT forum with a couple fixes and features added -- mainly the
generation of all the different image variants to support all known
models directly, atm known variants are AnnexA-WW, AnnexA-NA and
AnnexB-DE/GR.

I tested the patch successfully on my device.

Signed-off-by: Marco Antonio Mauro <marcus90@gmail.com>

SVN-Revision: 41236