Navigator: Add the ability to do a low-altitude RTL to avoid traffic

This is a first baseline implementation for collision avoidance with aircraft equipped with transponders. Since the drone-to-drone and drone-to-airplane system will be regulated by a standard, we do carefully avoid to do anything too fancy. This implementation allows to reduce altitude to get close to ground and land immediately or RTL. This is similar to what a human pilot would do and allows to get out of manned airspace which is at higher altitudes.

src/modules/navigator/navigator_params.c

@@ -139,6 +139,21 @@ PARAM_DEFINE_INT32(NAV_DLL_ACT, 0);
  */
 PARAM_DEFINE_INT32(NAV_RCL_ACT, 2);
 
+/**
+ * Set traffic avoidance mode
+ *
+ * Enabling this will allow the system to respond
+ * to transponder data from e.g. ADSB transponders
+ *
+ * @value 0 Disabled
+ * @value 1 Warn only
+ * @value 2 Return to Land
+ * @value 3 Land immediately
+ *
+ * @group Mission
+ */
+PARAM_DEFINE_INT32(NAV_TRAFF_AVOID, 1)
+
 /**
  * Airfield home Lat
  *