Adding a trace_mode to rfm2pi firmware

In an attempt to help with RFM network issues I've added a tool to a copy of the rfm2pi (rfm69) firmware to assist with diagnosing RF issues such as interference, heavy traffic etc

It uses the RFM69's RSSI to plot a real time (vertical) trace of the RF signal. the sample rate can be defined between 0.1s and 2.55s using the "y" as a command via a serial console. (eg 100y sets a 1 sample per second trace and 25y sets 4 samples per second)  

The group, quiet mode and frequency settings can be set and altered as usual to help locate an issue,

Although there is an issue in JeeLib with using "promiscuous mode" ie group 0, more on this can be found in the Data loss due to RF packets getting corrupted thread and I have enquired  over at JeeLabs about a fix.

Since it doesn't effect the normal running of the firmware when not enabled, I have submitted a pull request to the rfm2pi repo so hopefully it can get included as standard, emonhub can also be altered to ignore these lines and prevent loads of "non-numerical data" messages filling up the logfiles.