scd: Support ECC key generation.

* scd/app-openpgp.c (get_public_key): Fix a message.
(change_keyattr_from_string, ecc_writekey): Call mpi_release sooner.
(do_genkey): Add ECC support.

In OpenPGP card specification 3.0, ECC is introduced. So far, do_genkey
only supported RSA. Since KDF spec. is needed to calculate the
fingerprint, it is hard coded in app-openpgp.c. But it's defined by
OpenPGP ECC (RFC-6637), and card does nothing with KDF in fact.

• Co-authored-by: Arnaud Fontaine <arnaud.fontaine@ssi.gouv.fr>
• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>