Add compile-time, constant keystore callback

[billphipps]

Compile time keys should be allowed in the server and accessible as readonly on the server, identified by keyid's in the same namespace as cached/nvm keys.

1. Add a callback to check for keyids within the constant keystore when searching for a matching keyid within cache or nvm.
2. Add a callback to check for keyids within the constant keystore when searching for a unique keyid.
3. Optional: provide a fixed API and refactor existing key cache and nvm keystores to match that API
4. Keystore entries should store full metadata and variable length data.
5. Provide tool/headers to build keystore contents during compile time or to generate a separate binary file to be overlaid during link.