RC_Channel: allow for pass-thru from low channels to any channel

this makes it easier to setup things like ignition cut or bottle drop
with a 8 channel radio

libraries/RC_Channel/RC_Channel_aux.cpp

@@ -35,6 +35,9 @@ RC_Channel_aux::output_ch(void)
     case k_manual:              // manual
         radio_out = radio_in;
         break;
+    case k_rcin1 ... k_rcin8: // rc pass-thru
+        radio_out = hal.rcin->read(function-k_rcin1);
+        break;
     case k_motor1 ... k_motor8:
         // handled by AP_Motors::rc_write()
         return;
@@ -342,6 +345,10 @@ RC_Channel_aux::set_servo_limit(RC_Channel_aux::Aux_servo_function_t function, R
                 // also have to override radio_in
                 ch->radio_in = pwm;
             }
+            if (ch->function.get() >= k_rcin1 && ch->function.get() <= k_rcin8) {
+                // save for k_rcin*
+                ch->radio_in = pwm;
+            }
         }
     }
 }

libraries/RC_Channel/RC_Channel_aux.h

@@ -74,6 +74,14 @@ public:
         k_motor6                = 38,
         k_motor7                = 39,
         k_motor8                = 40,
+        k_rcin1                 = 51,            ///< these are for pass-thru from arbitrary rc inputs
+        k_rcin2                 = 52,
+        k_rcin3                 = 53,
+        k_rcin4                 = 54,
+        k_rcin5                 = 55,
+        k_rcin6                 = 56,
+        k_rcin7                 = 57,
+        k_rcin8                 = 58,
         k_nr_aux_servo_functions         ///< This must be the last enum value (only add new values _before_ this one)
     } Aux_servo_function_t;