Add LED_BUILTIN definition

Author: pennam

examples/timedBlink/timedBlink.ino

@@ -10,6 +10,10 @@
 
 #include <Arduino_TimedAttempt.h>
 
+#if !defined(LED_BUILTIN) && !defined(ARDUINO_NANO_ESP32)
+static int const LED_BUILTIN = 2;
+#endif
+
 TimedAttempt blink(500, 1000);
 
 void setup() {