AP_Airspeed: added last_update_ms() interface

libraries/AP_Airspeed/AP_Airspeed.cpp

@@ -212,4 +212,5 @@ void AP_Airspeed::read(void)
     _last_pressure          = airspeed_pressure;
     _raw_airspeed           = sqrtf(airspeed_pressure * _ratio);
     _airspeed               = 0.7f * _airspeed  +  0.3f * _raw_airspeed;
+    _last_update_ms         = hal.scheduler->millis();
 }

libraries/AP_Airspeed/AP_Airspeed.h

@@ -131,6 +131,9 @@ public:
     // return health status of sensor
     bool healthy(void) const { return _healthy; }
 
+    // return time in ms of last update
+    uint32_t last_update_ms(void) const { return _last_update_ms; }
+
     static const struct AP_Param::GroupInfo var_info[];
 
     enum pitot_tube_order { PITOT_TUBE_ORDER_POSITIVE =0, 
@@ -150,6 +153,7 @@ private:
     float			_last_pressure;
     float           _EAS2TAS;
     bool		    _healthy;
+    uint32_t        _last_update_ms;
 
     Airspeed_Calibration _calibration;
     float _last_saved_ratio;