Add ET-Board BUILTIN_LED pin (#5490)

ET-Board
No internal LED pin
Missing internal LED pin have been added to this commit.

variants/ET-Board/pins_arduino.h

@@ -11,6 +11,12 @@
 #define digitalPinToInterrupt(p)    (((p)<40)?(p):-1)
 #define digitalPinHasPWM(p)         (p < 34)
 
+static const uint8_t LED_BUILTIN = 5;
+#define BUILTIN_LED  LED_BUILTIN // backward compatibility
+#define LED_BUILTIN LED_BUILTIN
+
+
+
 static const uint8_t TX = 34;
 static const uint8_t RX = 35;