Add flushReceiveBuffer() to HardwareSerial

In Serial communication sometimes one wants to flush all data in the receive buffer at once. E.g. because data has become invalid. - https://forum.arduino.cc/index.php?topic=530650 -

The code to do this very fast in HardwareSerial is straightforward,
```
void HardwareSerial::flushReceiveBuffer()
{
_rx_buffer_head = _rx_buffer_tail;
}
```

首先定位 HardwareSerial 的声明及其现有的 flush 相关实现或测试。检查接收缓冲区的表示方式，以及如何测试公共 API 的新增内容；当 API 提供 flushReceiveBuffer() 且调用它会丢弃当前缓冲的所有接收数据时，即表示完成。

由索引模型根据 Issue 内容生成。