If the measurements are missing but it's a UEFI system, it's a good indication
that the user has secure boot turned off.
Notify the user on the UEFI device through a non-fatal `UpdateMessage`
To accomplish this, move fu-uefi-vars into the plugin library for other plugins to use
In theory, these should always match the reported PCRx values from the TPM.
If the reconstructed event log checksum does not match the TPM value then
something is either implemented wrongly, or something bad has happened.
This means we use half the amount of memory to store the event hashes, and also
means we can process the raw data in future patches without parsing back out
of ASCII format.
