kbuild: Create INSTALL_PATH directory if it does not exist

If INSTALL_PATH is not a valid directory, create it, like what
modules_install and dtbs_install will do in the same situation.

Signed-off-by: Zhang Bingwu <xtexchooser@duck.com>
Reviewed-by: Nicolas Schier <nicolas@jasle.eu>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>