说明：  实现类似于C语言标准函数printf()的简单printf打印。主要用于包含UART口的打印输出，代替标准的C库函数，减少对MCU的FLASH的占用，而又不失其基本功能，是在开发MCU时的好帮手。
(Simple printf to print to achieve is similar to the C language standard function printf (). Mainly used for the printout contains the UART ports, instead of the standard C library functions, reducing the occupancy of the MCU FLASH, but without losing its basic functions, is a good helper in the development of an MCU.)