AFAIK you can use GPLv3 for non-replaceable stuff. The thing is only to allow the users to replace it IIF it's phisically possible to do so. If you make a device that boots from a ROM it's not a problem. If you sign your updates and keep your public key on a ROM and there is no way to boot anything else… there's a problem.

> If you sign your updates and keep your public key on a ROM and there is no way to boot anything else… there's a problem.

As there should be.