tpm2-util: add helper that checks for the various facets of TPM2 support

tpm2-util: add helper that checks for the various facets of TPM2 support

So far we were a bit sloppy regarding checks for TPM2 support. Let's
make things more precise and introduce a single helper that checks for
three axis of TPM2 support: whether we have a loaded kernel driver,
whether the firmware used it, and whether we ourselves are compiled for
it.

This only adds the helper. Follow-up patches will use it at various
places.