[feat][examples/rtc] add rtc calendar case

examples/rtc/rtc_calendar/CMakeLists.txt

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+set(mains main.c)
+generate_bin()

examples/rtc/rtc_calendar/main.c

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+/**
+ * @file main.c
+ * @brief
+ *
+ * Copyright (c) 2021 Bouffalolab team
+ *
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.  The
+ * ASF licenses this file to you 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 "hal_rtc.h"
+#include "timestamp.h"
+
+int main(void)
+{
+    rtc_time beijing_time;
+
+    bflb_platform_init(0);
+    rtc_init(0);                   //set 0 means comparing none
+    rtc_set_timestamp(1628074512); //1628074512 :2021-8-4 18:55:12
+    while (1) {
+        bflb_platform_delay_ms(1000);
+        unixtime2bejingtime((uint32_t)rtc_get_timestamp(), &beijing_time);
+        MSG("%d/%02d/%02d-%02d:%02d:%02d convert is: %u\r\n",
+            beijing_time.year, beijing_time.month, beijing_time.day,
+            beijing_time.hour, beijing_time.minute, beijing_time.second, (uint32_t)rtc_get_timestamp());
+    }
+}