Fix some compilation warnings and add esp-idf platform for experiment (#454)

And fix some code indent issues.

product-mini/platforms/esp-idf/main.c

@@ -0,0 +1,39 @@
+/*
+ * Copyright (C) 2019 Intel Corporation.  All rights reserved.
+ * SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+ */
+
+#include "bh_platform.h"
+
+extern void
+iwasm_main(void);
+
+static void
+wamr_task(void *param)
+{
+    os_printf("WAMR task started\n");
+
+    iwasm_main();
+
+    while (1) {
+        task_sleep(1000);
+        /*os_printf("Hello WAMR\n");*/
+    }
+    (void)param;
+}
+
+static bool
+app_init(uint32_t id)
+{
+    os_printf("WAMR init, id: %d\n", id);
+    task_start("wamr_task", 8192, 4, wamr_task, NULL);
+    return true;
+}
+
+static void
+app_exit(uint32_t id)
+{
+    os_printf("WAMR exit, id: %d\n", id);
+}
+
+INTERNAL_APP_DEFINE("WAMR", APP_VERSION(0, 0, 0, 1), 0, app_init, app_exit);