SIM7020新增一个AT命令

根据sim7020代码自带的文档：LinkIt_for_RTOS_APB_Proxy_Developers_Guide.pdf特意做如下读书笔记

1、增加一个AT命令流程如下

1.1新增执行AT命令的代码，如下内容：

apb_proxy_status_t apb_proxy_example_at_command_handler ( apb_proxy_parse_cmd_param_t *p_parse_cmd )
{
	configASSERT ( p_parse_cmd != NULL );
	apb_proxy_at_cmd_result_t cmd_result;
	// Please note, if the output result is short, stack buffer also can be used.
	uint8_t *p_out_buffer = NULL;
	// Initialize cmd_result.
	cmd_result.result_code = APB_PROXY_RESULT_OK; // “OK” string is sent out by AP Bridge.
	cmd_result.pdata = NULL;
	cmd_result.length = 0;
	cmd_result.cmd_id = p_parse_cmd->cmd_id;

	switch ( p_parse_cmd->mode )
	{
		case APB_PROXY_CMD_MODE_READ:
			{
				p_out_buffer = ( uint8_t * ) pvPortMalloc ( buffer_size );
				memset ( p_out_buffer, 0, buffer_size );
				// Do something. Put the custom command result data into p_out_buffer.
				break;
			}

		case APB_PROXY_CMD_MODE_ACTIVE:
			{
				p_out