Update URL to obtain the repo tool

The repo tool's download location has changed. Update the README
to contain the new location. For reference, see the equivalent chagne
to Android and associated discussion:

https://android-review.googlesource.com/#/c/66864/
https://groups.google.com/forum/#!topic/repo-discuss/4EsDRDRK5Lk

Reported-by: kusma on IRC #tegra
Signed-off-by: Stephen Warren <swarren@nvidia.com>