Implement 2D graphic API (#87)

* Implement 2D graphic library based on LittlevGL

* Add lvgl license file

samples/gui/wasm-runtime-wgl/src/platform/zephyr/main.c

@@ -0,0 +1,37 @@
+/*
+ * Copyright (C) 2019 Intel Corporation.  All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include <stdlib.h>
+#include <string.h>
+#include "bh_platform.h"
+#include "wasm_assert.h"
+#include "wasm_log.h"
+#include "wasm_platform_log.h"
+#include "wasm_thread.h"
+#include "wasm_export.h"
+#include "wasm_memory.h"
+#include "bh_memory.h"
+
+extern int iwasm_main();
+
+void main(void)
+{
+    iwasm_main();
+    for(;;){
+        k_sleep(1000);
+    }
+}
+