AP_BoardConfig: add public method returning true if on sensor error

libraries/AP_BoardConfig/AP_BoardConfig.cpp

@@ -226,8 +226,11 @@ void AP_BoardConfig::init_safety()
 /*
   notify user of a fatal startup error related to available sensors. 
 */
+bool AP_BoardConfig::_in_sensor_config_error;
+
 void AP_BoardConfig::sensor_config_error(const char *reason)
 {
+    _in_sensor_config_error = true;
     /*
       to give the user the opportunity to connect to USB we keep
       repeating the error.  The mavlink delay callback is initialised

libraries/AP_BoardConfig/AP_BoardConfig.h

@@ -44,6 +44,9 @@ public:
 
     // notify user of a fatal startup error related to available sensors. 
     static void sensor_config_error(const char *reason);
+    // permit other libraries (in particular, GCS_MAVLink) to detect
+    // that we're never going to boot properly:
+    static bool in_sensor_config_error(void) { return _in_sensor_config_error; }
 
 #if CONFIG_HAL_BOARD == HAL_BOARD_PX4 || CONFIG_HAL_BOARD == HAL_BOARD_VRBRAIN
     // public method to start a driver
@@ -149,6 +152,8 @@ private:
 
 #endif // HAL_BOARD_PX4 || HAL_BOARD_VRBRAIN
 
+    static bool _in_sensor_config_error;
+
     // target temperarure for IMU in Celsius, or -1 to disable
     AP_Int8 _imu_target_temperature;
 };