Replace all menu_add_prop mimicking menu_add_prompt with the latter func. I've
had to add a return value to menu_add_prompt for one usage.

I've rebuilt scripts/kconfig/zconf.tab.c_shipped by hand to reflect changes
in the source (I've not the same Bison version so regenerating it wouldn't
have been not a good idea), and compared it with what Roman itself did some
time ago, and it's the same.

So I guess this can be finally merged.
Signed-off-by: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>