U-BOOT更新失败

[Michael.Zhu]

因为之前有个ddr和eth0的bug所以用新的debian image-69需要更新uboot和spl；
如果你已经刷uboot失败，除了说明书里面修复的章节外，其实还有个版本，如下：

1. flash sdcard.img into tf card;
2. power on and login system(user/password: root/starfive);
3. transfer the latest uboot/spl files into board by USB or Ethernet;
4. flashcp command to update;
5. re-insert the debian tf card and reboot;