[SGX] Build Migration Updates

[tqchen]

• CMake migration
• NNVM example migration

cc @nhynes now that nnvm is merged into tvm, we need to merge nnvm/examples/sgx to apps/sgx as a single example

[tqchen]

@nhynes can you act on this issue?

[nhynes]

hrm.. the current best way to use TVM in SGX is via Rust. The C++ is still segfaulting for unknown reasons. I could try to update the C++ version, but it's not the preferred interface imho. How would you like to proceed?

[tqchen]

Let us update the c++ interface to make sure it is complete, since there might still people who want to use it through python

[tqchen]

closed by #1763