Rework TPM measurements to use fewer PCRs. After discussion with upstream,
it's preferable to avoid using so many PCRs. Instead, measure into PCRs 8
and 9 but use a prefix in the event log to indicate which subsystem carried
out the measurements.
Add support for performing basic TPM measurements. Right now this only
supports extending PCRs statically and only on UEFI and BIOS systems, but
will measure all modules as they're loaded.