tegra2: Optimize out-of-tree build for Ventana.

As proposed by Mike Frysinger, mkdir can take more than one argument.
Instead of spawning two processes, create both the common and seaboard
directories in one go.

Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de>
Signed-off-by: Tom Warren <twarren@nvidia.com>