upate rootfs packet to buildroot version, update build shell script and add...

guowenxue

2019-08-12 a68e8c60fd9fa2ab9dc91312c6ba0d75302e92d7

linux-bsp/tarballs/linux-at91.tar.xz does not exist in tree 2c3f513ba5052988ab05d2ea78d0a9a7d44f97a2