https://qooah.com/2013/07/28/%e6%9c%89%e5%9c%96%e6%9c%89%e7%9c%9f%e7%9b%b8-%e6%96%b0%e4%b8%80%e4%bb%a3-nexus-7-%e6%88%90%e5%8a%9f%e8%a2%ab-root/