Add Gnuk in pkcs15 emulation layer.
Work-around for reading pubkey from Gnuk.

I apply workaround because the Gnuk support only short APDU and
the sc_get_response() does not work as expected for this case.
This workaround based on predicting response data length.
This just works with pubkey read, because Gnuk supports only 2048-bit key,
whose length is fixed.
This does not work with certficate read, because its length is variable.