ramips: add support for DuZun DM06

DuZun DM06 is a develop board based on mt7628
64M RAM, 8M SPI Flash, 1 WAN, 1 LAN.
wm8960 codec with line out, line in and speaker output.

Signed-off-by: Michael Lee <igvtee@gmail.com>

ramips: convert a bunch of targets to new image building code

Signed-off-by: Zoltan HERPAI <wigyori@uid0.hu>

ramips: move different subtarget image generation to separate makefiles to improve maintainability

- all subtarget specific defines are in their own files
- common defines left in main Makefile
- each subtarget makefile idefed with SUBTARGET
- all subtargets compile tested
- few seems to be broken/unneeded things marked with FIXME

Signed-off-by: Roman Yeryomin <roman@advem.lv>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@49104 3c298f89-4303-0410-b956-a3cf2f4a3e73