pcrlock: switch access policy for nvindex to store policy in from PolicyAuthValue...

pcrlock: switch access policy for nvindex to store policy in from PolicyAuthValue to PolicySigned (with an HMAC-SHA256 key)

So far the nvindex to store the pcrlock policy in was protected via a
PolicyAuthValue policy (i.e. with a simple PIN set on the nvindex).
That's a bad idea however, as it means an attacker can simply remove and
re-create the nvindex and the "name" of the nvindex does not change,
thus defeating the logic. (This is because the authValue is *not* part
of the "name" of an nvindex!).

Fix this by switching from PolicyAuthValue to PolicySigned with an
HMAC-SHA256 key. Behaviour is very similar: however, the PIN is now part
of of the access policy hash, which *is* part of the "name" of an
nvindex. Thus, if an attacker removes and recreates the nvindex it has
to provide the same PIN again or the "name" of the nvindex will change.
Mission accomplished.

I'd like to thank Chris Coulson for finding this issue (and helping me
address it). Thank you!