Add GPIO12 startup/activity heartbeat LED behavior

include/heartbeat.h

@@ -0,0 +1,11 @@
+#ifndef HEARTBEAT_H
+#define HEARTBEAT_H
+
+#include <Arduino.h>
+
+void setupHeartbeat();
+void startupHeartbeatBlink(uint8_t count = 5);
+void triggerHeartbeatBlink();
+void updateHeartbeat();
+
+#endif