kconfig: do not hardcode "include/config/auto.conf" filename

Regardless of KCONFIG_AUTOCONFIG, the filename written as a Make target
into "include/config/auto.conf.cmd" was always the default one.

Of course this doesn't make it work for the Kernel kbuild system, since
there the filename is hardcoded at several places in the Makefiles.

Signed-off-by: Markus Heidelberg <markus.heidelberg@web.de>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>