SGX: Establish secure session between enclave and tenant

[lkatalin]

After remote attestation is successful, the tenant must establish a secure communication channel with the enclave in order to provision secrets (i.e., the code to be run on Enarx). This is done through a DHKE. The public key for the enclave is:

1. generated inside the enclave

2. embedded in the Report Data portion of the enclave's Report, which is in turn embedded in the verified and signed Quote

Blocked by: mbedTLS modifications.