AP_GPS: prevent writing GPS log headings multiple times

libraries/AP_GPS/AP_GPS_SBP.cpp

@@ -47,6 +47,7 @@ extern const AP_HAL::HAL& hal;
 #define SBP_HW_LOGGING 0
 #endif
 
+bool AP_GPS_SBP::logging_started = false;
 
 AP_GPS_SBP::AP_GPS_SBP(AP_GPS &_gps, AP_GPS::GPS_State &_state, AP_HAL::UARTDriver *_port) :
     AP_GPS_Backend(_gps, _state, _port),
@@ -57,8 +58,7 @@ AP_GPS_SBP::AP_GPS_SBP(AP_GPS &_gps, AP_GPS::GPS_State &_state, AP_HAL::UARTDriv
     dops_msg_counter(0),
     baseline_msg_counter(0),
     crc_error_counter(0),
-    last_healthcheck_millis(0),
-    logging_started(false)
+    last_healthcheck_millis(0)
 {
 
     Debug("Initializing SBP Driver");

libraries/AP_GPS/AP_GPS_SBP.h

@@ -216,7 +216,7 @@ private:
     // ************************************************************************
 
     // have we written the logging headers to DataFlash?
-    bool            logging_started:1;
+    static bool logging_started;
 
     void logging_write_headers();
     void logging_log_health(float pos_msg_hz, float vel_msg_hz, float dops_msg_hz, float baseline_msg_hz, float crc_error_hz);

libraries/AP_GPS/AP_GPS_UBLOX.cpp

@@ -44,6 +44,8 @@ extern const AP_HAL::HAL& hal;
 #define UBLOX_HW_LOGGING 0
 #endif
 
+bool AP_GPS_UBLOX::logging_started = false;
+
 AP_GPS_UBLOX::AP_GPS_UBLOX(AP_GPS &_gps, AP_GPS::GPS_State &_state, AP_HAL::UARTDriver *_port) :
     AP_GPS_Backend(_gps, _state, _port),
     _step(0),

libraries/AP_GPS/AP_GPS_UBLOX.h

@@ -254,7 +254,7 @@ private:
     bool            need_rate_update:1;
 
     // have we written the logging headers to DataFlash?
-    bool            logging_started:1;
+    static bool     logging_started;
 
     uint8_t         _disable_counter;