Vendor pubkey protection

[mchack-work]

The coming vendor pubkey for Castor will live in the filesystem.

• Update Castor threat model about the vendor pubkey handling.
• Investigate need of downgrade attack protection
• Investigate need to protect vendor pubkey on flash, by for instance encrypting it with a key from boot verifier.
• Investigate if vendor pubkey should be updateable by anyone or if it's limited to whomever already have a pubkey installed.

[mchack-work]

To buy us time in this question we have decided to lock down the filesystem publisher pubkey to only be able to be updated by the current holder of the corresponding private key. See: tillitis/tkey-boot-verifier#35

This is not a final decision.