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
Let's make sure that the moment where factory reset is requested is
visible in the TPM PCR state, so that access to secrets is terminated.
This is particulary interesting when the system is booted with
systemd.unit=factory-reset.target on the kernel command line, requesting
a factory reset on the following boot. The preparations done in
userspace should already lose access to the TPM in that case.
