initdata: measure initdata digest into rt register

This adds two post exec directives for process-user-data in the mkosi to
extend PCR 8 (grub, which we don't use it mkosi podvms:
https://uapi-group.org/specifications/specs/linux_tpm_pcr_registry/)
with the digest of initdata. Sha256 and Sha384 banks are attempted.
initdata.digest contains a hex value that will fit only in one of those.
A failure of either post exec step will be ignored and do not turn the
unit status into a failure.

This is a bit provisional, but since things with init-data and runtime
measurement are a bit in flux still, which doesn't warrant to put such
logic in code yet.

Signed-off-by: Magnus Kulke <[email protected]>

src/cloud-api-adaptor/podvm-mkosi/mkosi.skeleton/usr/lib/systemd/system/process-user-data.d/10-override.conf

@@ -0,0 +1,3 @@
+[Service]
+ExecStartPost=-/bin/bash -c 'tpm2_pcrextend 8:sha256=$(cat /run/peerpod/initdata.digest)'
+ExecStartPost=-/bin/bash -c 'tpm2_pcrextend 8:sha384=$(cat /run/peerpod/initdata.digest)'