tpm2-common: break out pcr handling functions

To process creation data, the policy routines which collect PCR values
are broken out into a new function: tpm2_pcrs_get_digest(), which gets
all the selected PCRs into a TPML_DIGEST_VALUES structure (if not
NULL) and also returns the pcrDigest hash of all of them (for
comparison against the creationData.pcrDigest).  Also add ability to
specify empty PCR range (as "-") to tpm2_get_pcr_lock().

Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>