util/kconfig: Uprev to Linux 6.8's kconfig

Linux kconfig has its own implementation of KCONFIG_WERROR now, so use
that. This reduces our patch count by 2.

Change-Id: I4f5f1f552e96f8ef7a4c5c0ab2ab7e2b6d798ceb
Signed-off-by: Patrick Georgi <patrick@georgi.software>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/81223
Reviewed-by: Felix Singer <service+coreboot-gerrit@felixsinger.de>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

util/kconfig/util.c

@@ -42,8 +42,7 @@ struct gstr str_new(void)
 /* Free storage for growable string */
 void str_free(struct gstr *gs)
 {
-	if (gs->s)
-		free(gs->s);
+	free(gs->s);
 	gs->s = NULL;
 	gs->len = 0;
 }