[feat][examples/pka] add system prng demo

examples/pka/sec_trng_random/CMakeLists.txt

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

examples/pka/sec_trng_random/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_sec_ecdsa.h"
+#include <stdlib.h>
+
+extern void srandom(unsigned int __seed);
+extern long	random(void);
+int main(void)
+{
+    bflb_platform_init(0);
+
+    srandom(hal_get_trng_seed());
+    while (1) {
+        bflb_platform_printf("rand:%d\r\n", random());
+        bflb_platform_delay_ms(1000);
+    }
+}