u-root tpm2 PCR measurements are not robust to TPM Carte Blanche

[chrisfenner]

Describe the bug

I noticed pkg/tss only interacts with the SHA256 PCRs, and assumes that hashes being passed to the library are SHA256 hashes. Couple problems:

1. This code might run on a system whose TPM has a different PCR bank enabled instead (like SHA1 or SHA384)
2. This code might run on a system that has both SHA256 and another PCR algorithm enabled. This means that u-root will leave that other PCR bank totally empty and free for falsified attestations by later code.

A long-winded explanation of "TPM Carte Blanche" is available at https://www.dlp.rip/tpm-carte-blanche.

To Reproduce
n/a (issue reported based on inspection of the code)

Expected behavior

• u-root extends all PCR banks
• u-root doesn't assume every digest is a SHA256 digest

Additional context
https://www.dlp.rip/tpm-carte-blanche