Rover: cork and push servo outputs

SRV_Channels API makes it easy to sync all PWM channels at once. This is
the support needed for it to work properly on Rovers.

APMrover2/Steering.cpp

@@ -329,7 +329,9 @@ void Rover::set_servos(void) {
 #if HIL_MODE == HIL_MODE_DISABLED || HIL_SERVOS
     // send values to the PWM timers for output
     // ----------------------------------------
+    hal.rcout->cork();
     SRV_Channels::output_ch_all();
+    hal.rcout->push();
 #endif
 }