Attestation mechanism: SGX DCAP

- precondition for #15 to really be useful
- simplifies install procedure as there is now a debian repo which automatically updates the base packages and includes a driver which uses dkms to auto-rebuild on kernel upgrades. (Alas, last time i've checked, the sdk itself was still not yet a debian packages and is required even for just running FPC _iff_ you want to use simulation mode. This means also changing our docu from just pointing to SGX SDK source and more explicitly mention install options. See the [corresponding change in PDO](https://github.com/hyperledger-labs/private-data-objects/pull/229/files) as a starting point