libbladeRF: Suppress initial transients in loopback_onoff test

RX'ing data as the TX module turns on in loopback will yield some
transient noise, so both modules are enabled earlier in the program.

The RX task now waits until it sees 10 consecutive buffers of values
below the "ON" threshold before signaling the TX thread to start. This
ensures we ignore any transient spikes at occur after the FPGA has been
loaded.