IsoApplet: Simplify the private key import

Use the new features of sc_asn1_put_tag introduced in OpenSC/OpenSC#314.
Additionally, a RSA private key is sent from one large buffer using either extended APDUs or chaining (in compliance to IsoApplet API version 00.05).
IsoApplet: Don't set ECC field length of 512

512 is wrong for EC FP (correct would be 521 bit), and neither of those
two are currently supported by OpenSC.