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

firmware/baseband/proc_am_audio.cpp

@@ -108,7 +108,8 @@ void NarrowbandAMAudio::capture_config(const CaptureConfigMessage& message) {
 	}
 }
 
-void run() {
+int main() {
 	EventDispatcher event_dispatcher { std::make_unique<NarrowbandAMAudio>() };
 	event_dispatcher.run();
+	return 0;
 }