DataFlash: added logging_started() API

libraries/DataFlash/DataFlash.h

@@ -52,6 +52,8 @@ public:
     void Log_Write_Message(const char *message);
     void Log_Write_Message_P(const prog_char_t *message);
 
+    bool logging_started(void) const { return log_write_started; }
+
 	/*
       every logged packet starts with 3 bytes
     */
@@ -76,6 +78,7 @@ protected:
     const struct LogStructure *_structures;
     uint8_t _num_types;
     bool _writes_enabled;
+    bool log_write_started;
 
     /*
       read a block

libraries/DataFlash/DataFlash_Block.h

@@ -54,7 +54,6 @@ private:
     uint16_t df_Read_PageAdr;
     uint16_t df_FileNumber;
     uint16_t df_FilePage;
-    bool log_write_started;
 
     // offset from adding FMT messages to log data
     bool adding_fmt_headers;

libraries/DataFlash/DataFlash_File.cpp

@@ -349,6 +349,7 @@ void DataFlash_File::stop_logging(void)
     if (_write_fd != -1) {
         int fd = _write_fd;
         _write_fd = -1;
+        log_write_started = false;
         ::close(fd);
     }
 }
@@ -382,6 +383,7 @@ uint16_t DataFlash_File::start_new_log(void)
         return 0xFFFF;
     }
     _write_offset = 0;
+    log_write_started = true;
 
     // now update lastlog.txt with the new log number
     fname = _lastlog_file_name();