Configurable SGX signing key

**Is your feature request related to a problem? Please describe.**
Currently we generate on-demand new sgx signing keys for each enclave we generate (e.g., via
`common/crypto/Makefile`,
`ecc_enclave/enclave/CMakeLists-common-app-enclave.txt`, and
`tlcc_enclave/enclave/CMakeLists.txt`). For real/production use this would though have to be a single secure key (and whitelisted by Intel for EPID).

**Describe the solution you'd like**
Make it so that there is a common key used by the build scripts and this key can be configured as input and is only optionally created on the fly.

**Describe alternatives you've considered**

**Additional context**