Distributing the keys is illegal, but what about using they keys and distributing the resulting signed firmware?
Depending on how you look at it using another's entity's cryptographic key to sign something that then "pretends" to have been produced by that entity might classify as forgery.
Though this is just my personal thoughts, not sure if that would hold up in practice. Technically you own the hardware, so "forging" the signature yourself for your hardware probably wouldn't be an issue, but distributing it might be.
Perhaps, if you were attempting to make that claim to an actual person in a commercial context, deceiving them for personal gain. But in the context of "pretending" only to the hardware? Unlikely, as hardware has no standing in court. Even for-profit distribution shouldn't be an issue so long as it's clear to the human recipients that the signature is only present to fulfill technical requirements and the item wasn't actually produced by the other entity.
https://en.wikipedia.org/wiki/Google_LLC_v._Oracle_America,_...