Add M5Stack GREY module. (#189)

* Add M5Stack GREY module.

c.f. https://m5stack.com/collections/m5-core/products/grey-development-core

* Rename board.

* Fix build section.

Author: kisom

boards/m5stack-grey.json

@@ -0,0 +1,32 @@
+{
+  "build": {
+    "core": "esp32",
+    "extra_flags": "-DARDUINO_M5Stack_Core_ESP32",
+    "f_cpu": "240000000L",
+    "f_flash": "40000000L",
+    "flash_mode": "dio",
+    "ldscript": "esp32_out.ld",
+    "mcu": "esp32",
+    "variant": "m5stack_core_esp32"
+  },
+  "connectivity": [
+    "wifi",
+    "bluetooth",
+    "ethernet",
+    "can"
+  ],
+  "frameworks": [
+    "arduino",
+    "espidf"
+  ],
+  "name": "M5Stack GREY ESP32",
+  "upload": {
+    "flash_size": "16MB",
+    "maximum_ram_size": 532480,
+    "maximum_size": 16777216,
+    "require_upload_port": true,
+    "speed": 921600
+  },
+  "url": "http://www.m5stack.com",
+  "vendor": "M5Stack"
+}