debian 9 stretch arm v7 源

debian 9 stretch V7的中国源 原来总是拼不对，结果这样可以的

而实际上 ftp.cn.debian.org = mirrors.ustc.edu.cn

deb http://ftp.cn.debian.org/debian/ stretch main contrib non-free

deb http://ftp.cn.debian.org/debian/ stretch-updates main contrib non-free
deb http://mirrors.ustc.edu.cn/debian-security/ stretch/updates main contrib non-free
deb http://ftp.cn.debian.org/debian/ stretch-backports main contrib non-free