RC_Channel: added k_throttle

for secondary throttle on planes

libraries/RC_Channel/RC_Channel_aux.cpp

@@ -132,6 +132,10 @@ void RC_Channel_aux::aux_servo_function_setup(void)
         // tenth percentage tilt
         set_range_out(0,1000);
         break;
+    case RC_Channel_aux::k_throttle:
+        // fixed wing throttle
+        set_range_out(0,100);
+        break;
     default:
         break;
     }

libraries/RC_Channel/RC_Channel_aux.h

@@ -94,6 +94,7 @@ public:
         k_ignition              = 67,
         k_choke                 = 68,
         k_starter               = 69,
+        k_throttle              = 70,
         k_nr_aux_servo_functions         ///< This must be the last enum value (only add new values _before_ this one)
     } Aux_servo_function_t;