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>