Bugfix when using sonar as well as rx-parallel-pwm

This allows sonar to be used at the same time as parallel PWM for the receiver.  If the RX_PARALLEL_PWM feature is enabled, then the sonar system uses motor ports 5 and 6, otherwise it uses receiver pins 7 and 8.
Cleanup sonar support for Naze/Flip32/Olimexino.  Add Sonar
documentation.