card: Allow to store and retrieve keyblocks in OpenPGP cards.

* tools/gpg-card.c: Include tlv.h.
(cmd_writecert): Add option --openpgp.
(cmd_readcert): Ditto.

We use the CERT object for this and encapsulate the key block in a CMS
object.

• Signed-off-by: Werner Koch <wk@gnupg.org>