TPMTOTP HOTP code to be extended to 40 bytes (vs 20)

[tlaurion]

This is to track changes needed inside tpmtotp so that secret can be extended to use full 40 bytes available in HOTP in firmware

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@tlaurion Firmware supports it per your notes, but hotp from tpmtotp still requires exactly 20 bytes: https://github.com/osresearch/tpmtotp/blob/master/hotp.c#L26

The secret does not have to be printable, so I can use 20 binary bytes from the hash and get 160 bits of entropy, up from 80 currently (20 hex chars).

tpmtotp can be extended but I feel like this is going down a rabbit hole to go from 160 bits to 256 bits. Can we track that for an improvement later?

Originally posted by @JonathonHall-Purism in #1419 (comment)