global: Convert simple_strtoul() with hex to hextoul()

It is a pain to have to specify the value 16 in each call. Add a new
hextoul() function and update the code to use it.

Add a proper comment to simple_strtoul() while we are here.

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

cmd/rng.c

@@ -24,7 +24,7 @@ static int do_rng(struct cmd_tbl *cmdtp, int flag, int argc, char *const argv[])
 	}
 
 	if (argc >= 2)
-		n = simple_strtoul(argv[1], NULL, 16);
+		n = hextoul(argv[1], NULL);
 
 	buf = malloc(n);
 	if (!buf) {