kbuild: add kbuild-file macro

While building, installing, cleaning, Kbuild visits sub-directories
and includes 'Kbuild' or 'Makefile' that exists there.

Add 'kbuild-file' macro, and reuse it from scripts/Makefie.*

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>
Reviewed-by: Alexander Lobakin <alobakin@pm.me>
Tested-by: Alexander Lobakin <alobakin@pm.me>