Use channel 26 on all radios

Currently rfxlink radios are using mostly channel 11, and cc2420 uses channel 26. Therefor, they doesn't communicate with each other out of the box, which is a hard to detect "bug" for beginners.
Becouse cc2420 is the older stack, and channel 26 is the most wifi free channel*, I think we should switch to channel 26 on all capable hardware. Does anyone has a problem with this, or I can merge this?

* It overlaps with wifi channel 14, which is not available in most of the world, and slightly overlaps with wifi channel 13, which is not available in the US.