Now tcpip_output() is a function pointer that is supposed to be set via the macro tcpip_set_outputfunc(). Packet drivers do so on process startup.
Thus if there are several packet drivers in a Contiki system the one started last is the one actually used. This behaviour is especially useful for the 'IP forwarding' "meta" packet driver.
This change means that one needs to select _ONE_ cfs implementation at Contiki library link time. But this doesn't appear to be an issue as all platforms have their "favorite" implementation anyway.
