DataFlash: know about armed state of vehicle

libraries/DataFlash/DataFlash.cpp

@@ -72,6 +72,17 @@ void DataFlash_Class::setVehicle_Startup_Log_Writer(vehicle_startup_message_Log_
     _vehicle_messages = writer;
 }
 
+void DataFlash_Class::set_vehicle_armed(const bool armed_state)
+{
+    if (armed_state == _armed) {
+        // no change in status
+        return;
+    }
+
+    _armed = armed_state;
+}
+
+
 void DataFlash_Class::set_mission(const AP_Mission *mission) {
     FOR_EACH_BACKEND(set_mission(mission));
 }

libraries/DataFlash/DataFlash.h

@@ -205,6 +205,8 @@ public:
     bool logging_enabled() const;
     bool logging_failed() const;
 
+    void set_vehicle_armed(bool armed_state);
+
 protected:
 
     const struct LogStructure *_structures;
@@ -258,6 +260,8 @@ private:
     // fmt; includes the message header
     int16_t Log_Write_calc_msg_len(const char *fmt) const;
 
+    bool _armed;
+
 private:
     static DataFlash_Class *_instance;
 };