tools/atrf-txrx/atrf-txrx.c: new option -R for RTT measurement mode
tools/atrf-txrx/atrf-txrx.c: new option -R for RTT measurement mode

This is similar to ping (-P) but separates roles more clearly and
collects timing statistics instead of visualizing reception.

cfriedt/ben-wpan,cfriedt/ben-wpan,cfriedt/ben-wpan,cfriedt/ben-wpan,cfriedt/ben-wpan,cfriedt/ben-wpan