You don't need to upgrade the bootstrap/bootroms on a 2500.
Usually, it's recommanded to sync the bootimage with the main image (ie if you have 12.1(5) as the main image you should also use a 12.1(5) boot image). This is not the case for 25xx where bootimages are on a bootrom and are not meant to be synced with the main ios running on the router.