Add support for A71CH

- Add a71ch.py and a71ch_pubkey.py. These provide the same functionality
as the ecc608 variants.
- Modify core to check both security chips. If both are present, ecc608
would be used.
- Adjust debian dependencies to require either python3-cryptoauthlib, or
a71ch-crypto-support.
- Downgrade messages about not finding a crypto chip to debug, since it
is unlikely that you have both chips.

Change-Id: I09e04eb7025b19a0c8b5b4e608f4e7d4605c9d19