kbuild: always run 'make silentoldconfig' when tree is cleaned

If the file .kconfig.d is missing then make sure to run
'make silentoldconfig', since we have no way to detect if
a Kconfig file has been updated.

-kconfig.d is created by kconfig and is removed as part
of 'make clean' so the situation is likely to occur in reality.

Jan Beulich <JBeulich@novell.com> reported this bug.

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