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/disk/part.c b/disk/part.c
index 086da84..a6a8f70 100644
--- a/disk/part.c
+++ b/disk/part.c
@@ -396,7 +396,7 @@
 		hwpart = 0;
 	}
 
-	dev = simple_strtoul(dev_str, &ep, 16);
+	dev = hextoul(dev_str, &ep);
 	if (*ep) {
 		printf("** Bad device specification %s %s **\n",
 		       ifname, dev_str);
@@ -405,7 +405,7 @@
 	}
 
 	if (hwpart_str) {
-		hwpart = simple_strtoul(hwpart_str, &ep, 16);
+		hwpart = hextoul(hwpart_str, &ep);
 		if (*ep) {
 			printf("** Bad HW partition specification %s %s **\n",
 			    ifname, hwpart_str);
@@ -534,7 +534,7 @@
 		part = PART_AUTO;
 	} else {
 		/* Something specified -> use exactly that */
-		part = (int)simple_strtoul(part_str, &ep, 16);
+		part = (int)hextoul(part_str, &ep);
 		/*
 		 * Less than whole string converted,
 		 * or request for whole device, but caller requires partition.