kconfig: move .kernelrelease

This moves the .kernelrelease file into include/config directory.  Remove its
generation from the config step, if the config step doesn't leave a proper
.config behind, it triggers a call to silentoldconfig.  Instead its generation
can be done via proper dependencies.

Signed-off-by: Roman Zippel <zippel@linux-m68k.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>