CLEANUP: config: Return ERR_NONE from config callbacks instead of 0

Return ERR_NONE instead of 0 on success for all config callbacks that should
return ERR_* codes. There is no change because ERR_NONE is a macro equals to
0. But this makes the return value more explicit.
diff --git a/src/wdt.c b/src/wdt.c
index d6a3e56..18836ff 100644
--- a/src/wdt.c
+++ b/src/wdt.c
@@ -174,7 +174,7 @@
 	sigemptyset(&sa.sa_mask);
 	sa.sa_flags = SA_SIGINFO;
 	sigaction(WDTSIG, &sa, NULL);
-	return 0;
+	return ERR_NONE;
 }
 
 REGISTER_POST_CHECK(init_wdt);