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/cmd/broadcom/nitro_image_load.c b/cmd/broadcom/nitro_image_load.c
index 4a36b30..93b5cb4 100644
--- a/cmd/broadcom/nitro_image_load.c
+++ b/cmd/broadcom/nitro_image_load.c
@@ -53,13 +53,13 @@
 		return CMD_RET_USAGE;
 
 	/* convert command parameter to fastboot address (base 16), i.e. hex */
-	images_load_addr = simple_strtoul(argv[1], NULL, 16);
+	images_load_addr = hextoul(argv[1], NULL);
 	if (!images_load_addr) {
 		pr_err("Invalid load address\n");
 		return CMD_RET_USAGE;
 	}
 
-	spi_load_addr = simple_strtoul(argv[2], NULL, 16);
+	spi_load_addr = hextoul(argv[2], NULL);
 	if (!spi_load_addr) {
 		pr_err("Invalid spi load address\n");
 		return CMD_RET_USAGE;