kbuild: fix building with redirected output.

Jan Altenberg <jan.altenberg@linutronix.de> reported that
building with redirected input like this failed:
make O=dir oldconfig bzImage < /dev/null

The problem were caused by a make silentoldconfig being
run before oldconfig and with a non-recent .config the build
failed because silentoldconfig requires non-redirected stdin.

Silentoldconfig was run as a side-effect of having the
top-level Makefile re-made by make.
Introducing an empty rule for the top-level Makefile
(and Kbuild.include) fixed the issue.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>