BC28发送AT+CIMI命令前要加个2S延时,不然容易报错

       'rt_thread_mdelay(2000);
        /* check IMSI */
        if (at_obj_exec_cmd(device->client, resp, "AT+CIMI") != RT_EOK)
        {
            result = -RT_ERROR;
            LOG_E(">> AT+CIMI");
            goto __exit;
        }