kconfig: obey KCONFIG_ALLCONFIG choices with randconfig.

Currently when using KCONFIG_ALLCONFIG with randconfig the choice options
are clobbered.  As recommended by Roman, this adds an is_new test to see
whether to select a new option or obey the existing one.

This is a resend of the earlier patch a couple of weeks ago, since there
was no reply.  Original thread is at http://lkml.org/lkml/2007/11/28/94

Signed-off-by: Paul Mundt <lethal@linux-sh.org>
Cc: Roman Zippel <zippel@linux-m68k.org>
Cc: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>