STM32重定义printf，实现串口打印
在“usart.c”文件中加入以下代码
#ifdef __GNUC__
#define PUTCHAR_PROTOTYPE int __io_putchar(int ch)
#else
#define PUTCHAR_PROTOTYPE int fputc(int ch, FILE *f)
#endif
PUTCHAR_PROTOTYPE
{
HAL_UART_Transmit(&huart1 , (uint8_t *)&ch, 1, 0xFFFF);
return ch;
}