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>
diff --git a/board/ti/j721e/evm.c b/board/ti/j721e/evm.c
index 580f13c..077d834 100644
--- a/board/ti/j721e/evm.c
+++ b/board/ti/j721e/evm.c
@@ -201,7 +201,7 @@
 	if (env_get("serial#"))
 		return;
 
-	board_serial = simple_strtoul(ep->serial, &endp, 16);
+	board_serial = hextoul(ep->serial, &endp);
 	if (*endp != '\0') {
 		pr_err("Error: Can't set serial# to %s\n", ep->serial);
 		return;