kconfig: save location of config symbols

When we add a new config symbol save the file/line
so we later can refer to their location.

The information is saved as a property to a config symbol
because we may have multiple definitions of the same symbol.

This has the side-effect that a symbol always has
at least one property.

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