kconfig: Add wildcard support for "source"

Kconfig's include directive "source" does not support
wildcards (e.g. source src/mainboard/*/Kconfig) which
makes automatic inclusion of all boards a tedious task
and prevents us from implementing "drop in" boards.

In our Makefile.inc files we already include mainboard
directories per wildcard, so let's add the infrastructure
to do the same with Kconfig.

Change-Id: I1988ff6ce3e167e86bb5cb65fc04a13748599dad
Signed-off-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
Reviewed-on: http://review.coreboot.org/9294
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

util/kconfig/lkc.h

@@ -78,6 +78,7 @@ void zconf_starthelp(void);
 FILE *zconf_fopen(const char *name);
 void zconf_initscan(const char *name);
 void zconf_nextfile(const char *name);
+void zconf_nextfiles(const char *name);
 int zconf_lineno(void);
 const char *zconf_curname(void);