Update Arduino core to v3.0.2

Resolve #255

Author: valeros

boards/heltec_wifi_kit_8.json

@@ -4,12 +4,12 @@
       "ldscript": "eagle.flash.4m1m.ld"
     },
     "core": "esp8266",
-    "extra_flags": "-DESP8266 -DARDUINO_ARCH_ESP8266 -DARDUINO_ESP8266_HELTEC_WIFI_KIT_8",
+    "extra_flags": "-DESP8266 -DARDUINO_ARCH_ESP8266 -DARDUINO_wifi_kit_8",
     "f_cpu": "80000000L",
     "f_flash": "40000000L",
     "flash_mode": "dio",
     "mcu": "esp8266",
-    "variant": "d1_mini"
+    "variant": "wifi_kit_8"
   },
   "connectivity": [
     "wifi"

platform.json

@@ -53,7 +53,7 @@
       "type": "framework",
       "optional": true,
       "owner": "platformio",
-      "version": "~3.30001.0"
+      "version": "~3.30002.0"
     },
     "framework-esp8266-rtos-sdk": {
       "type": "framework",