pefile: Parse the presumed PKCS#7 content of the certificate blob

Parse the content of the certificate blob, presuming it to be PKCS#7 format.

Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Vivek Goyal <vgoyal@redhat.com>
Reviewed-by: Kees Cook <keescook@chromium.org>