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>
Allow --boards to add to not replace the set of enabled boards

The following already work:

Build all boards that are enabled by default:
./build build

Build a specific set of boards:
./build --boards beaver,jetson-tk1 build

Sometimes it's nice to build all boards that are enabled by default,
plus a few others which aren't. This patch enables the following syntax
for that:
./build --boards +jetson-tk1 build

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Merge pull request #1 from NVIDIA/master

#1