The previous commit largely optimized the object directory creation.
We can optimize it more for incremental build.
There are already *.cmd files in the output directory. The existing
*.cmd files have been picked up by $(wildcard ...). Obviously,
directories containing them exist too, so we can skip "mkdir -p".
With this, Kbuild runs almost zero "mkdir -p" in incremental building.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
