Make generate_enclave.sh more robust

Currently, `ecc_enclave` and `tlcc_enclave` use a script to generate `mrenclave.go`. However, this script is a) duplicated and b) not robust.

As suggested by @g2flyer ...

... bigger problem seems that we do no catch any errors and so it was happily running so far (just producing only an empty mrenclave). Easiest fix for that seems to be o just add a 'set -e' in the beginning?

Also, given that the generate_mrenclave.sh is the same in ecc_enclave and tlcc_enclave, maybe we should move that to a common bin directory?

Thinking of it, the method here also seems rather brittle, the way we do it in [pdo ](https://github.com/hyperledger-labs/private-data-objects)is more robust:

add to sgx_sign sign the '-dumpfile "${SIGNED_ENCLAVE_METADATA}"' option (see [https://github.com/hyperledger-labs/private-data-objects/blob/master/common/CMakeLists.txt](common/CMakeLists.txt)) and then
extract it with a more understandable like the ' VAR_MRENCLAVE=$$(perl -0777 -ne 'if (/metadata->enclave_css.body.enclave_hash.m:([a-fx0-9 \n]+)/) { $$eh = $$1; $$eh=~s/0x| |\n//g; $$eh=~tr/a-z/A-Z/; print "$${eh}\n"; }' $(CONTRACT_ENCLAVE_MRENCLAVE_META_FILE))' in target '$(CONTRACT_ENCLAVE_MRENCLAVE_C_FILE)' in [pservice/Makefile](https://github.com/hyperledger-labs/private-data-objects/blob/master/pservice/Makefile)