global: Convert simple_strtoul() with decimal to dectoul()

It is a pain to have to specify the value 10 in each call. Add a new
dectoul() function and update the code to use it.

Signed-off-by: Simon Glass <sjg@chromium.org>

cmd/exit.c

@@ -11,7 +11,7 @@ static int do_exit(struct cmd_tbl *cmdtp, int flag, int argc,
 		   char *const argv[])
 {
 	if (argc > 1)
-		return simple_strtoul(argv[1], NULL, 10);
+		return dectoul(argv[1], NULL);
 
 	return 0;
 }