scd: GENKEY updates the public key in APP.

* scd/app-openpgp.c (rsa_read_pubkey, ecc_read_pubkey): New.
(read_public_key): New.
(get_public_key, do_genkey): Use read_public_key.

With this change, since GENKEY updates the public key (pk[keyno].key) in
APP, READKEY will be possible after the command even for the old
card (version <= 0x0100).

• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>