add TODO for saola on-board neopixel

hw/bsp/esp32s2_saola_1/esp32s2_saola_1.c

@@ -32,6 +32,7 @@
 // MACRO TYPEDEF CONSTANT ENUM DECLARATION
 //--------------------------------------------------------------------+
 
+// TODO use saola-1 on-board neopixel (WS2812)
 #define LED_PIN               21
 
 #define BUTTON_PIN            0