Update Arduino core to v3.1.0

Resolves #290

Author: valeros

boards/d1_wroom_02.json

@@ -0,0 +1,32 @@
+{
+  "build": {
+    "arduino": {
+      "ldscript": "eagle.flash.2m64.ld"
+    },
+    "core": "esp8266",
+    "extra_flags": "-DESP8266 -DARDUINO_ARCH_ESP8266 -DARDUINO_ESP8266_WEMOS_D1WROOM02",
+    "f_cpu": "80000000L",
+    "f_flash": "40000000L",
+    "flash_mode": "dio",
+    "mcu": "esp8266",
+    "variant": "d1_mini"
+  },
+  "connectivity": [
+    "wifi"
+  ],
+  "frameworks": [
+    "arduino",
+    "esp8266-rtos-sdk",
+    "esp8266-nonos-sdk"
+  ],
+  "name": "WeMos D1 ESP-WROOM-02",
+  "upload": {
+    "maximum_ram_size": 81920,
+    "maximum_size": 2097152,
+    "require_upload_port": true,
+    "resetmethod": "nodemcu",
+    "speed": 115200
+  },
+  "url": "https://artofcircuits.com/product/esp-wroom-02-wifi-18650-battery-charger-board",
+  "vendor": "WeMos"
+}

platform.json

@@ -49,7 +49,7 @@
       "type": "framework",
       "optional": true,
       "owner": "platformio",
-      "version": "~3.30002.0"
+      "version": "~3.30100.0"
     },
     "framework-esp8266-rtos-sdk": {
       "type": "framework",