KEYS: Add parser for TPM-based keys [ver #2]

For TPM based keys, the only standard seems to be described here:
http://david.woodhou.se/draft-woodhouse-cert-best-practice.html#rfc.section.4.4



Quote from the relevant section:
"Rather, a common form of storage for "wrapped" keys is to encode the
binary TCPA_KEY structure in a single ASN.1 OCTET-STRING, and store the
result in PEM format with the tag "-----BEGIN TSS KEY BLOB-----". "

This patch implements the above behavior.  It is assumed that the PEM
encoding is stripped out by userspace and only the raw DER/BER format is
provided.  This is similar to how PKCS7, PKCS8 and X.509 keys are
handled.

Signed-off-by: Denis Kenzior <denkenz@gmail.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Tested-by: Marcel Holtmann <marcel@holtmann.org>
Reviewed-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: James Morris <james.morris@microsoft.com>