Capture: Write simple metadata for baseband and audio files.

Will probably turn the audio metadata file into .wav data, eventually.

firmware/application/analog_audio_app.hpp

@@ -181,6 +181,7 @@ private:
 	bool is_recording() const;
 	void record_start();
 	void record_stop();
+	void write_metadata_file(const std::string& filename);
 };
 
 } /* namespace ui */