It is not Tivoization. Tivoization is when device manufacturers deliberately lock down their devices so the user cannot change the code even though it is open source.
What's the difference whether your kernel doesn't boot due to signature checks or due to lack of essential drivers?
You might be able to write the drivers without running afoul of the law. If it's a signature check, breaking it could be construed as a DMCA violation (in the US, probably some other law in friendly countries). Neither one's great, but there is a difference.