DataFlash: implement a printf-like Log_Write_MessageF method

libraries/DataFlash/DataFlash.cpp

@@ -7,6 +7,8 @@
 
 DataFlash_Class *DataFlash_Class::_instance;
 
+extern const AP_HAL::HAL& hal;
+
 const AP_Param::GroupInfo DataFlash_Class::var_info[] = {
     // @Param: _BACKEND_TYPE
     // @DisplayName: DataFlash Backend Storage type
@@ -251,6 +253,18 @@ bool DataFlash_Class::logging_failed() const
     return backends[0]->logging_failed();
 }
 
+void DataFlash_Class::Log_Write_MessageF(const char *fmt, ...)
+{
+    char msg[64] {};
+
+    va_list ap;
+    va_start(ap, fmt);
+    hal.util->vsnprintf(msg, sizeof(msg), fmt, ap);
+    va_end(ap);
+
+    Log_Write_Message(msg);
+}
+
 #define FOR_EACH_BACKEND(methodcall)              \
     do {                                          \
         for (uint8_t i=0; i<_next_backend; i++) { \

libraries/DataFlash/DataFlash.h

@@ -129,6 +129,7 @@ public:
     bool Log_Write_MavCmd(uint16_t cmd_total, const mavlink_mission_item_t& mav_cmd);
     void Log_Write_Radio(const mavlink_radio_t &packet);
     void Log_Write_Message(const char *message);
+    void Log_Write_MessageF(const char *fmt, ...);
     void Log_Write_CameraInfo(enum LogMessages msg, const AP_AHRS &ahrs, const AP_GPS &gps, const Location &current_loc);
     void Log_Write_Camera(const AP_AHRS &ahrs, const AP_GPS &gps, const Location &current_loc);
     void Log_Write_Trigger(const AP_AHRS &ahrs, const AP_GPS &gps, const Location &current_loc);