kconfig: add a symbol string expansion helper

Signed-off-by: Arnaud Lacombe <lacombar@gmail.com>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Michal Marek <mmarek@suse.cz>

scripts/kconfig/lkc_proto.h

@@ -28,6 +28,7 @@ P(symbol_hash,struct symbol *,[SYMBOL_HASHSIZE]);
 
 P(sym_lookup,struct symbol *,(const char *name, int flags));
 P(sym_find,struct symbol *,(const char *name));
+P(sym_expand_string_value,const char *,(const char *in));
 P(sym_re_search,struct symbol **,(const char *pattern));
 P(sym_type_name,const char *,(enum symbol_type type));
 P(sym_calc_value,void,(struct symbol *sym));