Attest: Add key selection and modify key loading

Load the attestation key in token creation, and keep it loaded until the
end of token operation. Add functions to get public and private
attestation key.

Change-Id: Ia5b4a17cb0835a49b086bd94b72457f69ca66577
Signed-off-by: Raef Coles <raef.coles@arm.com>
4 files changed