arm: async echo app

arm/trace.h

@@ -0,0 +1,17 @@
+#pragma once
+
+#include <cstdint>
+
+namespace tracing {
+enum class TraceEvent : uint8_t {
+    kUnknown = 0,
+
+    kUartIsr,
+    kUartRxCb,
+    kUartTxCb,
+};
+
+void trace(TraceEvent event);
+void trace(int raw_event);
+void dump();
+}  // namespace tracing