Support for accessing material stored in a local TPM

[chucklever]

Some authentication material (eg, x.509 certificates) might be stored in a local TPM rather than in files. For example, if TLS is used to protect traffic to and from the block device on which the root filesystem is stored, keeping certificates in the root filesystem is probably not feasible.

To use a TPM, some mechanism might access the TPM and move the needed authentication material into a keyring that can then be accessed by tlshd.