Rework board detection, separate board specific code into its own file. As a
result we also change the way rdc images are generated.
Support for board which required binary tools, like AMIT are dropped. Patch by
Bernhard Loos.

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@20294 3c298f89-4303-0410-b956-a3cf2f4a3e73