Support for Measured boot with TPM

[TobiPeterG]

To improve the experience for users of encrypted root partitions, measured boot should be supported. This allows the TPM to unlock the root drive, when the kernel, initrd, cmdine and firmware are in a predicted state. The actual state would have to be measured at boot time. systemd-stub supports this, but I'm not sure about Lanzaboote

There are already tools to predict the PCR values, e.g. pcr-oracle, though it's not available for nixos yet
This also supports multiple entries and a secure system boot doesn't depend on secure boot anymore but the actual state of the boot files.

In a perfect world, an encrypted system would use the predictions to auto unlock on boot in case the files haven't been tempered with.