Since Ventana is derived from Seaboard and requires seaboard.c to build,
make sure board/nvidia/seaboard is created in the build tree.
Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de> Acked-by: Stephen Warren <swarren@nvidia.com> Tested-by: Stephen Warren <swarren@nvidia.com> Tested-by: Wolfgang Denk <wd@denx.de>