kconfig: always write out .config

Always write out .config also in the case where config
did not change.
This fixes: http://bugzilla.kernel.org/show_bug.cgi?id=11230

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Josh Boyer <jwboyer@linux.vnet.ibm.com>
Cc: Adrian Bunk <bunk@kernel.org>