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/boot.c

@@ -29,7 +29,7 @@ static int do_go(struct cmd_tbl *cmdtp, int flag, int argc, char *const argv[])
 	if (argc < 2)
 		return CMD_RET_USAGE;
 
-	addr = simple_strtoul(argv[1], NULL, 16);
+	addr = hextoul(argv[1], NULL);
 
 	printf ("## Starting application at 0x%08lX ...\n", addr);