PCR extensions are supposed to be useful for "destroying" the ability to
access TPM bound secrets. Hence, if for some reason we fail to extend a
PCR, it's safer to just reboot, instead of going on without the
extension, leaving secrets potentially accessible which should not be
accessible.
Note that the services exit gracefully if no TPM is found, hence this
should not be triggered on TPM-less systems. However, this enforces that
if there is a TPM that is accessible to Linux and that works properly,
the PCR measurement must complete too.
Inspired by this thread:
https://lists.freedesktop.org/archives/systemd-devel/2025-March/051244.html
storagetm mode means we we are network accessible. let's lock down
access to TPM secrets in this case: let's measure a pcr "phase" string
into PCR 11.
This is good as it means that if we are exploited in this state FDE
secrets protected by TPM are likely to remain protected, since the PCR
values wouldn't allow access.
