= add support for selectable 16MHz or 8MHz operation.
- running 3.3v@16MHz is technically "out of spec", should be 3.3v@8MHz or 5.0v@16MHz. but, the rx3s oscillator and fuses are set up for 16MHz default.
- now, the board target in the arduino IDE can either be
1. Arduino Pro or Pro Mini (5V, 16MHz) w/ ATmega 168
2. Arduino Pro or Pro Mini (3.3V, 8MHz) w/ ATmega 168
and the device will set the operating speed accordingly.

= relocate dump_sensors() later in the file (after copy_rx_in(), etc) to avoid needed some forward references, since dump_sensors() calls those functions.