added a 'multikiss' interface, supporting up to 2 KISS interfaces on one node:

* use handles to identify a specific multikiss interface
* use handle-specific rx/tx callbacks for plugging into USART drivers
* to retrieve a handle's interface, use csp_multikiss_get_if(handle)
* this also leaves the original kiss interface in tact, to avoid lots of code
  changes in other modules