增加uart1

使用方法（PB6 = TX，PB7 = RX）
rt_device_t device = rt_device_find("uart1");
if (device != RT_NULL)
rt_device_open(device, RT_DEVICE_OFLAG_RDWR);
rt_device_read(device,0,buf,len);
rt_device_write(device,0,buf,len);