kbuild: also delete temporary directories

Reuse the standard naming schema for temporary files also for temporary
directories.

Such a directory will be used by the kheaders generation.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>
Tested-by: Nicolas Schier <nicolas@fjasle.eu>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>