Add support for Jetson TK1 board

This board is currently disabled by default due to lack of U-Boot support
in the mainline repository. If you wish to test this, do as follows:

cd ../u-boot
git checkout some-branch-with-jetson-tk1-support
cd ../scripts
./build --boards venice2 build

Signed-off-by: Stephen Warren <swarren@nvidia.com>