kconfig: correct oldconfig for unset choice options

oldconfig currently ignores unset choice options and doesn't ask for them.
Correct the SYMBOL_DEF_USER flag of the choice symbol to be only set if
it's set for all values.

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