Describes the serial port under Linux

代码说明：

介绍了Linux下的串口驱动的设计层次及接口, 并指出串口与TTY终端之间的关联层次(串口可作TTY终端使用)-Describes the serial port under Linux-driven design and interface level, and noted that the serial link between the terminal and the TTY level (TTY serial port can be used for end-use)