tpm2: whenever we measure, also write a tpm log record

tpm2: whenever we measure, also write a tpm log record

Previously we only logged our measurements to the journal. This is not a
great solution though, since regular logs are subject to rotation, which
is something we really cannot have for measurements (as it means we can
never reproduce the PCR values from the data). Hence, let's maintain an
explicit log.

Ideally, we'd just use the TCG Canonical Event Log format 1:1
(https://trustedcomputinggroup.org/resource/canonical-event-log-format/).
However it's not a perfect fit fo us, for various reasons. But let's
follow it (in its JSON incantation) as closely at it makes sense, so
that it can easily be converted to the full format by programs consuming
it.

Code comments explain where we deviate from the TCG CEL-JSON, and what
to do about it when reading the data.