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/ubifs.c b/cmd/ubifs.c
index a26b653..7a620c5 100644
--- a/cmd/ubifs.c
+++ b/cmd/ubifs.c
@@ -122,14 +122,14 @@
 	if (argc < 3)
 		return CMD_RET_USAGE;
 
-	addr = simple_strtoul(argv[1], &endp, 16);
+	addr = hextoul(argv[1], &endp);
 	if (endp == argv[1])
 		return CMD_RET_USAGE;
 
 	filename = argv[2];
 
 	if (argc == 4) {
-		size = simple_strtoul(argv[3], &endp, 16);
+		size = hextoul(argv[3], &endp);
 		if (endp == argv[3])
 			return CMD_RET_USAGE;
 	}