Encryption key related improvements (QOL)

[811Alex]

Adds a table of known keys, associated with (partial) signatures.
When the signature of the current config.bin matches one in the table and the --key parameter is omitted, the associated key gets used automatically.
Also adds a new parameter, --try-all-known-keys, to decode.py, which tries all the keys in the table against the payload, until one decrypts it successfully (checks if the decrypted payload starts with the payload magic), or it runs out of keys.

Lastly, it adds a few messages to help the user.
If a key was automatically chosen, it gets printed, so the user knows how to re-encrypt the file.
If the decrypted payload doesn't start with the magic, the user gets an error suggesting that the key was wrong, instead of a stacktrace.
If encode.py gets used without a key or signature, the user gets some warnings.

[mkst]

Thanks for these improvements!