Configurable SGX signing key

[g2flyer]

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