kbuild: use 'output' variable to create the output directory

$(KBUILD_OUTPUT) specifies the output directory of kernel builds.

Use a more generic name, 'output', to better reflect this code hunk in
the context of external module builds.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>