SecurityPkg/Pkcs7Verify: Complete the Pkcs7VerifyDxe protocol
    
    VerifySignature can be implemented using a mirror of the
    AuthenticodeVerify function that's already in use in the
    ImageVerificationDXE environment, so this patch simply wires up
    VerifySignature using that code.
    
           this VerifySignature() implementation now.)
    
    Contributed-under: TianoCore Contribution Agreement 1.1
    Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>
    Reviewed-by: Long Qin <qin.long@intel.com>