pcrlock: generate recovery PINs via make_recovery_key()

pcrlock: generate recovery PINs via make_recovery_key()

We already have infrastructure for generating nice recovery keys, for
the usual cryptenroll recovery keys. Let's reuse them here, as they are
nicer to read and type than the base64 encoded randomness we so far
used.

Previously valid recovery keys remain valid, in their original format.
For future enrollments we'll however have nicer, easier recovery keys to
deal with.