kbuild: create a build directory automatically for out-of-tree build

Prior to Kbuild, the build system created a build directory,
when it did not exist, for out-of-tree build.

This feature was dropped when we switched to Kbuild
because many of lines in makefiles were copied from Linux Kernel.
(In Linux Kernel, we have to create a build directory by ourselves
before starting build.)

That feature seems worth reviving for less typing
even if our code and Linux Kernel diverge.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Suggested-by: Simon Glass <sjg@chromium.org>
Acked-by: Simon Glass <sjg@chromium.org>
Tested-by: Simon Glass <sjg@chromium.org>