RC_Channel: added disable_aux_channel()

this will be used by copter to ensure motor channels are not enabled
for aux functions

libraries/RC_Channel/RC_Channel.h

@@ -118,9 +118,11 @@ private:
     int16_t         _low;
     int16_t         _high_out;
     int16_t         _low_out;
-    uint8_t         _ch_out;
 
     static RC_Channel *rc_ch[RC_MAX_CHANNELS];
+
+protected:
+    uint8_t         _ch_out;
 };
 
 // This is ugly, but it fixes poorly architected library

libraries/RC_Channel/RC_Channel_aux.cpp

@@ -38,6 +38,20 @@ RC_Channel_aux::output_ch(unsigned char ch_nr)
     hal.rcout->write(ch_nr, radio_out);
 }
 
+/*
+  prevent a channel from being used for auxillary functions
+  This is used by the copter code to ensure channels used for motors
+  can't be used for auxillary functions
+*/
+void RC_Channel_aux::disable_aux_channel(uint8_t channel)
+{
+    for (uint8_t i = 0; i < RC_AUX_MAX_CHANNELS; i++) {
+        if (_aux_channels[i] && _aux_channels[i]->_ch_out == channel) {
+            _aux_channels[i] = NULL;
+        }
+    }    
+}
+
 /// Update the _aux_channels array of pointers to rc_x channels
 /// This is to be done before rc_init so that the channels get correctly initialized.
 /// It also should be called periodically because the user might change the configuration and

libraries/RC_Channel/RC_Channel_aux.h

@@ -100,6 +100,9 @@ public:
     // assigned and enable auxillary channels
     static void enable_aux_servos(void);
     
+    // prevent a channel from being used for auxillary functions
+    static void disable_aux_channel(uint8_t channel);
+
 private:
     static uint32_t _function_mask;
     static RC_Channel_aux *_aux_channels[RC_AUX_MAX_CHANNELS];