Add Range-based RTL

msg/CMakeLists.txt

@@ -108,6 +108,7 @@ set(msg_files
 	rc_channels.msg
 	rc_parameter_map.msg
 	rpm.msg
+	rtl_flight_time.msg
 	safety.msg
 	satellite_info.msg
 	sensor_accel.msg

msg/rtl_flight_time.msg

@@ -0,0 +1,5 @@
+uint64 timestamp				# time since system start (microseconds)
+
+uint8 rtl_vehicle_type				# from the vehicle_status message
+float32 rtl_time_s				# how long in seconds will the RTL take
+float32 rtl_limit_fraction			# what fraction of the allowable RTL time would be taken