ramips: use MkImage for Hauppauge Broadway, Netcore NW718 and Tenda W306R V2.0

These targets need custom image names specified in the uImage header.
MkImage accepts an optional 4th argument to do exactly that. Reuse the
facility to generate factory images.

Compile-tested, the generated files inspected with "file" utility.

[juhosg:
 - keep the factory.bin images to avoid users confusion,
 - use GENERIC_4M template for the W306R V2.0
 - update commit log]

Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Signed-off-by: Gabor Juhos <juhosg@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@35243 3c298f89-4303-0410-b956-a3cf2f4a3e73