AP_GPS: log ublox sw and hw versions on dataflash log start

This burns 41 bytes of RAM for each u-Blox GPS.

libraries/AP_GPS/AP_GPS_UBLOX.cpp

@@ -810,11 +810,13 @@ AP_GPS_UBLOX::_parse_gps(void)
             break;
         case MSG_MON_VER:
             _have_version = true;
+            strncpy(_version.hwVersion, _buffer.mon_ver.hwVersion, sizeof(_version.hwVersion));
+            strncpy(_version.swVersion, _buffer.mon_ver.swVersion, sizeof(_version.swVersion));
             GCS_MAVLINK::send_statustext_all(MAV_SEVERITY_INFO, 
                                              "u-blox %d HW: %s SW: %s",
                                              state.instance + 1,
-                                             _buffer.mon_ver.hwVersion,
+                                             _version.hwVersion,
-                                             _buffer.mon_ver.swVersion);
+                                             _version.swVersion);
             break;
         default:
             unexpected_message();
@@ -1319,3 +1321,15 @@ float AP_GPS_UBLOX::get_lag(void) const
         break;
     };
 }
+
+void AP_GPS_UBLOX::Write_DataFlash_Log_Startup_messages() const
+{
+    AP_GPS_Backend::Write_DataFlash_Log_Startup_messages();
+
+    if (_have_version) {
+        gps._DataFlash->Log_Write_MessageF("u-blox %d HW: %s SW: %s",
+                                           state.instance+1,
+                                           _version.hwVersion,
+                                           _version.swVersion);
+    }
+}

libraries/AP_GPS/AP_GPS_UBLOX.h

@@ -114,6 +114,7 @@ public:
     }
 
     void broadcast_configuration_failure_reason(void) const override;
+    void Write_DataFlash_Log_Startup_messages() const override;
 
     // return velocity lag
     float get_lag(void) const override;
@@ -522,6 +523,7 @@ private:
     uint8_t         _next_message;
     uint8_t         _ublox_port;
     bool            _have_version;
+    struct ubx_mon_ver _version;
     uint32_t        _unconfigured_messages;
     uint8_t         _hardware_generation;