kconfig: oldconfig shall not set symbols if it does not need to

Avoid setting the value if the symbol doesn't need to be changed or can't
be changed. Later choices may change the dependencies and thus the
possible input range.

make oldconfig from a 2.6.22 .config with CONFIG_HOTPLUG_CPU not set
was in some configurations setting CONFIG_HOTPLUG_CPU=y without asking,
even when there was no actual requirement for CONFIG_HOTPLUG_CPU.
This was triggered by SUSPEND_SMP that does a select HOTPLUG_CPU.

Signed-off-by: Roman Zippel <zippel@linux-m68k.org>
Tested-by: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>