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>
diff --git a/board/nvidia/ventana/Makefile b/board/nvidia/ventana/Makefile
index eced323..e3b7435 100644
--- a/board/nvidia/ventana/Makefile
+++ b/board/nvidia/ventana/Makefile
@@ -25,8 +25,7 @@
 include $(TOPDIR)/config.mk
 
 ifneq ($(OBJTREE),$(SRCTREE))
-$(shell mkdir -p $(obj)../seaboard)
-$(shell mkdir -p $(obj)../common)
+$(shell mkdir -p $(obj)../common $(obj)../seaboard)
 endif
 
 LIB	= $(obj)lib$(BOARD).o