util/kconfig: Uprev to Linux 6.2's kconfig

The upstream build system uses a newly introduced function `read-file`,
so copy that in from Linux 6.2.

TEST=`util/abuild/abuild -C` output (config.h and config.build) remains
the same

Change-Id: Ic100bf189ebd3eaa0eb26904ae8602910329a180
Signed-off-by: Patrick Georgi <patrick@coreboot.org>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/79179
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Martin L Roth <gaumless@gmail.com>

util/kconfig/lkc.h

@@ -80,7 +80,7 @@ struct gstr str_new(void);
 void str_free(struct gstr *gs);
 void str_append(struct gstr *gs, const char *s);
 void str_printf(struct gstr *gs, const char *fmt, ...);
-const char *str_get(struct gstr *gs);
+char *str_get(struct gstr *gs);
 
 /* menu.c */
 void _menu_init(void);