Makefile: Add mkdir dependency for each rule that outputs into directory

This prevents races with mkdir. While here, be explicit about which
directory we need to build by splitting up the mkdir rule.

Reported-by: Denys Dmytriyenko <denis@denix.org>
Signed-off-by: Andrew Davis <afd@ti.com>
Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>