geofence: added FENCE_TRIGGERED_PIN

this allows for a digital pin to be set high when the geofence
triggers. This pin can be connected to an external failsafe device

ArduPlane/config.h

@@ -809,3 +809,9 @@
 #ifndef FENCE_ENABLE_PWM
 # define FENCE_ENABLE_PWM 1750
 #endif
+
+// a digital pin to set high when the geo-fence triggers. Defaults
+// to -1, which means don't activate a pin
+#ifndef FENCE_TRIGGERED_PIN
+# define FENCE_TRIGGERED_PIN -1
+#endif

ArduPlane/geofence.pde

@@ -209,6 +209,9 @@ static void geofence_check(bool altitude_check_only)
             // we have moved back inside the fence
             geofence_state->fence_triggered = false;
             gcs_send_text_P(SEVERITY_LOW,PSTR("geo-fence OK"));
+#if FENCE_TRIGGERED_PIN > 0
+            digitalWrite(FENCE_TRIGGERED_PIN, LOW);
+#endif
         }
         // we're inside, all is good with the world
         return;
@@ -227,6 +230,11 @@ static void geofence_check(bool altitude_check_only)
     geofence_state->breach_count++;
     geofence_state->breach_time = millis();
     geofence_state->breach_type = breach_type;
+
+#if FENCE_TRIGGERED_PIN > 0
+    digitalWrite(FENCE_TRIGGERED_PIN, HIGH);
+#endif
+
     gcs_send_text_P(SEVERITY_LOW,PSTR("geo-fence triggered"));
 
     // see what action the user wants

ArduPlane/system.pde

@@ -224,6 +224,13 @@ static void init_ardupilot()
 #if CONFIG_RELAY == ENABLED
 	DDRL |= B00000100;					// Set Port L, pin 2 to output for the relay
 #endif
+
+#if FENCE_TRIGGERED_PIN > 0
+    pinMode(FENCE_TRIGGERED_PIN, OUTPUT);
+    digitalWrite(FENCE_TRIGGERED_PIN, LOW);
+#endif
+
+
 	// If the switch is in 'menu' mode, run the main menu.
 	//
 	// Since we can't be sure that the setup or test mode won't leave