allow --boards/--socs to undisable boards

Before this change, --boards/--socs only disabled more boards than were
disabled in their configuration files. Now, those options explicitly set
all boards/socs disabled state to match the --boards/socs option, if
given.

This is useful so that configurations may be added for boards that are
not yet fully supported by projects such as U-Boot; the configuration
can be added, but marked as disabled by default so that builds are not
broken, but then explicitly re-enabled by those who wish to test local
patches to enable that board in the other projects.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Add config files for Venice2 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-venice2-support
cd ../scripts
./build --boards venice2 build

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