EK abstraction: enable reading nonce and template from NV index

[THS-on]

The EK Credential profile spec also specifies a way of reading storing nonces and the EK template in NV indices. Because this feature is rarely used, can be manually implemented when needed, we don't have a good way of testing it right now, this was not implemented in #552.

This is mainly to document the difference from what is supported by tpm2-tools, which implements support for it.