SecurityPkg/Pkcs7Verify: Add the comments to address security problem

SecurityPkg/Pkcs7Verify: Add the comments to address security problem

Add the comments to address security problems in the Pkcs7Verify Protocol
per UEFI 2.7 updates.

The Pkcs7Verifier function VerifySignature() has problematic use cases
where it might be used to unwittingly bypass security checks.  The specific
problem is that if the supplied hash is a different algorithm from the
blacklist hash, the hash will be approved even if it should have been
denied. The added comments place a strong warning about the problem.
It is possible to use the protocol reliably, either by agreeing a hash to
use for all time (like sha256) or by looping over all supported hashes when
using the protocol.

Cc: Chao Zhang <chao.b.zhang@intel.com>
Cc: James Bottomley <James.Bottomley@HansenPartnership.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Qin Long <qin.long@intel.com>
Reviewed-by: Chao Zhang <chao.b.zhang@intel.com>