kbuild: fix make incompatibility

"Paul Smith" <psmith@gnu.org> reported that we would fail
to build with a new check that may be enabled in an
upcoming version of make.

The error was:

      Makefile:442: *** mixed implicit and normal rules.  Stop.

The problem is that we did stuff like this:

config %config: ...

The solution was simple - the above was split into two with identical
prerequisites and commands.
With only three lines it was not worth to try to avoid the duplication.

Cc: "Paul Smith" <psmith@gnu.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>