ChannelSpectrumConfig message subsumes FIFONotify.

Separate channel spectrum config from spectrum data. This will permit sending config info only when necessary.
Use type information of ChannelSpectrum to statically define number of FFT bins elsewhere.
TODO: Posting configuration message way too often. Fixing that is the next step.

firmware/baseband/spectrum_collector.hpp

@@ -60,6 +60,7 @@ private:
 	uint32_t channel_filter_stop_frequency { 0 };
 
 	void post_message(const buffer_c16_t& data);
+	void post_configuration_message();
 };
 
 #endif/*__SPECTRUM_COLLECTOR_H__*/