ARM: tegra: fix Ventana standalone build

ARM: tegra: fix Ventana standalone build

Ventana always pulls in files from the Seaboard directory, so needs to
mkdir $(obj)../seaboard unconditionally. This fixes:

git clean -f -d -x
./MAKEALL ventana

"MAKEALL -s tegra20" passes without this change, because Seaboard
happens to be built before Ventana, and hence the directory has already
been created.

I believe the mkdir is only needed for out-of-tree builds, since the
seaboard directory is part of the source tree. However, since we always
build an SPL for Tegra now, which I believe is effectively an out-of-tree
build, we will always need this at some time. The overhead of just
uncondtionally executing the mkdir is minimal, and simplifies the
Makefile, since we don't need to code up the exact minimal condition to
execute the mkdir.

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