If Software Freedom Conservancy win their lawsuit against Vizio for GPL violations in their TVs, you will probably be able to install open source Linux distros with Kodi on any Vizio TV and soon afterwards lots of other smart TV vendors will be similar. Allowing the vendor operating system to remain on the device after you purchase it basically means spyware these days.
I don't think the GPL covers signing keys, just some mechanism for installation. Allowing addition of additional signing keys is a good way to do it, just like UEFI Secure Boot does things. There are also plenty of phone vendors who allow bootloader unlock after wiping the device DRM/etc keys too.
https://sfconservancy.org/copyleft-compliance/vizio.html