kconfig: ignore select of unknown symbol

We have had warnings for a long time about select of unknow symbol
but the warnings does not really makes sense since we may
select a symbol that is relevant and defined in one
arch but not in another arch.
And as long as we do not use a common set of Kconfig files
for all archs lets just ignore this case.

Previously we have used this to find bad uses of
select but we need a more relaible method to do so.

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