kconfig: allow multiple default values per symbol

Extend struct symbol to allow storing multiple default values, which can be
used to hold multiple configurations.

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