runonphone: Implement traceswitch support for coda

runonphone: Implement traceswitch support for coda

The traceswitch application redirects debug messages to the USB
port. This was supported by the TRK backend but not by the CODA
backend.

1. CODA backend now recognises mux channel 2 as valid
2. new TextTraceHandler class to attach to channel 2 and decode
   trace packets / print them to stdout
3. CODA signal handler creates a TextTraceHandler & destroys it
   on shutdown

Reviewed-By: Miikka Heikkinen