ramips: use new image build code for D-Link DCS-930 family

ramips: use new image build code for D-Link DCS-930 family

Drop the factory images and the firmware tool to create them. They don't
work any more, since the factory image has an uImage header covering the
whole kernel + rootfs. This way the uImage splitter will not be able to
find the rootfs and the kernel will panic later on.

The factory images were most likely added at a time the board had
distinct partitions for kernel and rootfs.

Signed-off-by: Mathias Kresin <dev@kresin.me>
(backported from 262b885b834cecf7e7afbaca32d0a67c0b75aa44)