tegra20: make board mkdir commands unconditional

tegra20: make board mkdir commands unconditional

Change the mkdir commands for the object directories to be
unconditional.  This fixes an issue when building for SPL where
SRCTREE and OBJTREE are the same, but $(obj) is under SPLTREE.

Signed-off-by: Allen Martin <amartin@nvidia.com>
Acked-by: Stephen Warren <swarren@wwwdotorg.org>
Tested-by: Thierry Reding <thierry.reding@avionic-design.de>
Signed-off-by: Tom Warren <twarren@nvidia.com>