libavb_atx: Implement authenticated unlock

The ATX reference code implements two operations:
 - Generation of an unlock challenge.
 - Validation of an unlock credential provided as a response to the most
   recent challenge.

Additionally, avbtool has been enhanced to produce unlock credentials.

Bug: 72234756
Test: Unit tests

Change-Id: Ic837fdae07461332311164ac7cccf2fe463dd1e3