update linux-bsp build shell script, fix u-boot update crosstool bug

guowenxue

2019-08-27 69d0972fa8b82065a67566b74ecae8585d9c68c1

linux-bsp/tarballs/linux-at91.tar.xz does not exist in tree 984e2b76e4fd20e812b30c2439cd54dc25ed329a