Rev 0 of Stick-and-rudder via Dedicated RC Link Complete

Up until today, stick-and-rudder control has been a very complicated kludge:

RC transmitter -> USB buddy-box-to-joystick-converter cable -> PC -> windows joystick driver  -> pygame ->  joystick-reading code (Python) -> TCP socket client (Python) -> wired ethernet -> dedicated wifi router -> 'copter flight computer wifi link -> TCP socket server and control loop (Python) -> servo controller board -> motor speed controllers.

Sounds like absolute madness, but implementing this took very little time. It's always kinda sucked, though, and controlling the quadcopter felt really spongy and difficult. Lot's of crashing into the bushes and whatnot.

I've now got throttle control working via the RC radio link:

RC transmitter -> RC reciever (on 'copter) PPM out -> 'copter flight computer -> ppm_decoder and TCP socket client (in C) ->  TCP socket server and control loop (Python) -> servo controller board -> motor speed controllers.

Much simpler.

Observations:

- Throttle response is way faster. This makes me happy (like "I've got a boner" happy). 

- Output from ppm_server is very noisy. At half-throttle with no stick movement, the throttle value jumps around as much as +-20% of max-throttle. Watching the PPM signal with the o-scope, I don't see any such jumpiness. I suspect that we're up against interrupt latency. Time for a RT_PREEMPT kernel! (I've always wanted and excuse to try this).

- Whenever the radio is powered, the motors twitch, even though the throttle commands from hl_controller.py are zeroed. I suspect RF interference with the servo controller board/wiring. Sorting this one out with require more quality time with the o-scope.