[rtduino][stm32-l476] add LED_BUILTIN

bsp/stm32/stm32l476-st-nucleo/applications/arduino_pinout/pins_arduino.h

@@ -42,6 +42,8 @@
 
 #define F_CPU          80000000L  /* CPU:80MHz */
 
+#define LED_BUILTIN     D13  /* Default Built-in LED */
+
 /* i2c1 : PB9-SDA PB8-SCL */
 #define RTDUINO_DEFAULT_IIC_BUS_NAME     "i2c1"