ixp4xx: Add back support for Freecom FSG-3

ixp4xx: Add back support for Freecom FSG-3

OpenWrt supported the Freecom FSG-3 in the past. It has
64 MB of RAM so will run fine, but the bare 4 MB of flash
makes it a non-default target. The generated compressed
image is currently below 4MB (just 3.3 MB) though, so it
should be possible to flash just fine with a rootfs on
a harddrive or USB stick, which is what the FSG-3 used
in the past as well.

The device has a WAN port on eth0 and three LAN ports on
eth1. The LAN ports are probably a DSA switch but the
old OpenWrt base never activated that, instead it relies
on boot defaults.

Due to questionable usablity without tweaking and further
work this image is not built by default, but made available
for developers who know what they are doing.

The TAR+CRC image generation is a rewritten version of the
earlier support code.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>