Sorry to sound like a broken record on this post, but Mikrotik does this. Everything can be flashed. Bootloader and OS. You can even get into a bricked unit via

Sorry to sound like a broken record on this post, but Mikrotik does this. Everything can be flashed. Bootloader and OS. You can even get into a bricked unit via MAC address and their Netinstall utility, and wipe the whole thing with a clean image.