refactor logger: use static fields & move them to source file

avoids multiple declarations of...

src/modules/logger/logger.cpp

@@ -75,6 +75,10 @@
 
 using namespace px4::logger;
 
+static Logger *logger_ptr = nullptr;
+static int logger_task = -1;
+static pthread_t writer_thread;
+
 int logger_main(int argc, char *argv[])
 {
 	// logger currently assumes little endian
@@ -464,7 +468,7 @@ void Logger::run()
 		return;
 	}
 
-	int ret = _writer.thread_start(_writer_thread);
+	int ret = _writer.thread_start(writer_thread);
 
 	if (ret) {
 		PX4_ERR("logger: failed to create writer thread (%i)", ret);
@@ -616,7 +620,7 @@ void Logger::run()
 	_writer.notify();
 
 	// wait for thread to complete
-	ret = pthread_join(_writer_thread, NULL);
+	ret = pthread_join(writer_thread, NULL);
 
 	if (ret) {
 		PX4_WARN("join failed: %d", ret);

src/modules/logger/logger.h

@@ -203,11 +203,8 @@ private:
 	param_t						_log_utc_offset;
 	orb_advert_t					_mavlink_log_pub = nullptr;
 	uint16_t					_next_topic_id; ///< id of next subscribed topic
+
 };
 
-Logger *logger_ptr = nullptr;
-int		logger_task = -1;
-pthread_t _writer_thread;
-
-}
-}
+} //namespace logger
+} //namespace px4