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/armflash.c

@@ -280,7 +280,7 @@ static int do_afs(struct cmd_tbl *cmdtp, int flag, int argc, char *const argv[])
 	} else if (argc == 4 && !strcmp(argv[1], "load")) {
 		ulong load_addr;
 
-		load_addr = simple_strtoul(argv[3], NULL, 16);
+		load_addr = hextoul(argv[3], NULL);
 		ret = load_image(argv[2], load_addr);
 	} else {
 		return CMD_RET_USAGE;