OneShot mode for servo outputs

This adds support for the new OneShot125 servo mode. Normal ESCs use 1000..2000µs @ max. 400Hz for signal transfer. The new Ultra- and KISS-ESC use OneShot125 without a fixed rate. The ESC uses 8MHz instead of 1MHz for pulse width measuring and gets 950 internal steps. The OneShot is coupled with the PID update rate. The benefit is: There is no jitter between servo rate and sensor rate. The update rate can be up to 2kHz in OneShot according to the ESC description. I've tested it up to 3,9kHz without flying.