Specify proper LED pin for ESP-Mx DevKit

Fixes #289

Author: valeros

boards/espmxdevkit.json

@@ -4,7 +4,12 @@
       "ldscript": "eagle.flash.1m256.ld"
     },
     "core": "esp8266",
-    "extra_flags": "-DESP8266 -DARDUINO_ARCH_ESP8266 -DARDUINO_ESP8266_ESP01",
+    "extra_flags": [
+      "-DESP8266",
+      "-DARDUINO_ARCH_ESP8266",
+      "-DARDUINO_ESP8266_ESP01",
+      "-DLED_BUILTIN=16"
+    ],
     "f_cpu": "80000000L",
     "f_flash": "40000000L",
     "flash_mode": "dout",