scd:piv: Add feature to read Yubikey attestation certificates.

* scd/app-piv.c (do_readcert): Add hack to read Yubikey attestaions.

Use

gpg-card 'readcert PIV.ATST.9A >x.crt'

to store the attestation certificate for 9A into X.CRT.

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