Extend slip-radio example for Econotag and Nooliberry

This allows to build slip-radio for Econotag and Nooliberry. 
The changes were necessary because 802.15.4 channel selection is not part of the Radio API.

btw, is there an follow-up to [this discussion](http://comments.gmane.org/gmane.os.contiki.devel/14462)? any plans for radio_driver evolutions?