optionally run a software TPM at boot as fallback for TPM less machines (#41016)

In various scenarios it's useful to be able to run a software TPM as
fallback on a machine if it doesn't natively have a hardware for it, in
order to provide somewhat systematic interfacing for legacy machines.
This adds the infrastructure for it.

Relevant parts:

1. On EFI systems sd-stub will now generate a random secret on first
boot and store it in a persistent NV variable, which is marked
inaccessible to the OS. It then derives a per-OS secret from that which
is passed to the OS via the initrd logic. This is generally useful, but
in particular is intended to secure the software TPM at least a bit: it
provides better security than nothing (i.e. the only protection in place
is that the firmwrae protections work, but this is also what shim relies
on, hence maybe not too bad), and allows swtpm to encrypt its storage
with something.

2. systemd-tpm2-generator is extended to optionally start an swtpm if no
tpm hw is detected. Because this is of course a major downgrade in
security, this has to be requested explicitly at boot via a kernel
cmdline switch.

3. This optionally mounts the ESP from the initrd. This is general
infrastructure, and has been requested before, but is particularly
interesting in the context of software TPMs: the state needs to be
stored somewhere, and that before the rootfs can be unlocked.

4. This introduces a special new separator measurement for PCRs 0-7 that
isolates all measurements from pre-os/uefi world from those done by the
OS. I added this for three reasons:
- in the swtpm case we'll not have any pre-os/uefi measurements, and we
need to be able to determine cleanly that this is the case. this hence
is supposed to play a similar role as the usual firmware separator
measurement, that however cleanly fixates to the PCRs even the case
where the firmware measurements don't exist at all
   - this is a very comprehensive fix for #40567
- not all firmwares generate the firmware separator at all, but it is
essential to seal off firmware variables from OS generated ones. This
can fill the void to some degree.
6. This introduces a new kernel cmdline switch
systemd.tpm2-measured-os=1, which allows force enabling all our
measurement logic, even if UKI TPM measurements are not done. This is
supposed to be used for the swtpm case so that one gets all the
measurements even without having the early boot verified boot chain in
place.

Benefits of all this: systems that care about TPMs have a (lower
security) compat glue in place that allows supporting legacy hw the same
way as modern hw in many ways, so that remote attestation and other uses
can reasonably work with the same codepaths.

Also see: https://github.com/lxc/incus-os/pull/667 regarding prior
similar work.

Fixes: #40567