pcrphase: add two additional phases

pcrphase: add two additional phases

This adds two more phases to the PCR boot phase logic: "sysinit" +
"final".

The "sysinit" one is placed between sysinit.target and basic.target.
It's good to have a milestone in this place, since this is after all
file systems/LUKS volumes are in place (which sooner or later should
result in measurements of their own) and before services are started
(where we should be able to rely on them to be complete).

This is particularly useful to make certain secrets available for
mounting secondary file systems, but making them unavailable later.

This breaks API in a way (as measurements during runtime will change),
but given that the pcrphase stuff wasn't realeased yet should be OK.