keep arduino incompatible BUILTIN_LED

Author: tuxedo0801

variants/thing/pins_arduino.h

@@ -44,6 +44,8 @@ static const uint8_t SCK   = 14;
 
 static const uint8_t LED_BUILTIN = 5;
 
+static const uint8_t BUILTIN_LED = 5;
+
 static const uint8_t A0 = 17;