Replace getrandom with arc4random_buf to simplify the usage

and follow the kernel side change:
apache/nuttx#14509

Signed-off-by: Xiang Xiao <xiaoxiang@xiaomi.com>

fsutils/mkgpt/mkgpt.c

@@ -130,13 +130,7 @@ static const uint8_t g_partition_type_swap[16] =
 
 static void get_uuid(FAR uint8_t *uuid)
 {
-  /* call getrandom to read  /dev/urandom */
-
-  if (getrandom(uuid, 16, 0) < 0)
-    {
-      fprintf(stderr, "error read primary partition table\n");
-      return;
-    }
+  arc4random_buf(uuid, 16);
 }
 
 static void init_mbr(FAR uint8_t *mbr, uint32_t blocks)

netutils/dhcpc/dhcpc.c

@@ -520,10 +520,6 @@ FAR void *dhcpc_open(FAR const char *interface, FAR const void *macaddr,
   struct sockaddr_in addr;
   struct timeval tv;
   int ret;
-  const uint8_t default_xid[4] =
-  {
-    0xad, 0xde, 0x12, 0x23
-  };
 
   ninfo("MAC: %02x:%02x:%02x:%02x:%02x:%02x\n",
         ((uint8_t *)macaddr)[0], ((uint8_t *)macaddr)[1],
@@ -544,19 +540,7 @@ FAR void *dhcpc_open(FAR const char *interface, FAR const void *macaddr,
        * used by another client.
        */
 
-#if defined(CONFIG_DEV_URANDOM) || defined(CONFIG_DEV_RANDOM)
-      ret = getrandom(pdhcpc->xid, 4, 0);
-      if (ret != 4)
-        {
-          ret = getrandom(pdhcpc->xid, 4, GRND_RANDOM);
-          if (ret != 4)
-            {
-              memcpy(pdhcpc->xid, default_xid, 4);
-            }
-        }
-#else
-      memcpy(pdhcpc->xid, default_xid, 4);
-#endif
+      arc4random_buf(pdhcpc->xid, 4);
 
       pdhcpc->interface = interface;
       pdhcpc->maclen    = maclen;