Clean up baseband processors to each have a main().

firmware/baseband/proc_nfm_audio.cpp

@@ -96,7 +96,8 @@ void NarrowbandFMAudio::capture_config(const CaptureConfigMessage& message) {
 	}
 }
 
-void run() {
+int main() {
 	EventDispatcher event_dispatcher { std::make_unique<NarrowbandFMAudio>() };
 	event_dispatcher.run();
+	return 0;
 }