RFM69 compatability

rfm69 packets can be recognised and processed, automatically strips 'OK'
& RSSI from frame to pass for decoding and then appends the RSSI, minus
the brackets,  back to the frame of data before posting to buffer.
changed _validate_frame() to return frame rather than true to
accommodate rfm69.
Since the rfm12 runs at 9600 and the rfm69 at 38400 the default value of
9600 must be overridden with " com_baud = 38400 " in [[RFM2Pi]]
[[[runtime_settings]]] of emonhub.conf.