can't verify ECDSA signature signed by ecc_handle

In my enclave, I create a keypair and sign a message:
```
let ecc_handle = SgxEccHandle::new();
let _result = ecc_handle.open();
let (prv_k, pub_k) = ecc_handle.create_key_pair()?;
info!(" [Enclave] Generate ephemeral ECDSA keypair successful");
debug!(" pubkey X is {:02x}", pub_k.gx.iter().format(""));
debug!(" pubkey Y is {:02x}", pub_k.gy.iter().format(""));
...
info!(" [Enclave] sign ed25519 pubkey with ephemeral ECDSA key");
debug!(" payload {:02x}", chain_signer.public().0.iter().format(""));
let gxgy = ecc_handle
.ecdsa_sign_slice(&chain_signer.public().0, &prv_k)
.sgx_error()?;
let chain_signer_attestation = [gxgy.x, gxgy.y].concat();
let mut chain_signer_attestation_out = [0u32; 16];
chain_signer_attestation_out.copy_from_slice(&chain_signer_attestation[..]);
debug!(" signature: {:02x}", chain_signer_attestation_out.iter().format(""));
```
giving me
```
[2020-04-26T08:19:15Z INFO substratee_worker_enclave::attestation] [Enclave] Generate keypair
[2020-04-26T08:19:15Z INFO substratee_worker_enclave::attestation] [Enclave] Generate ephemeral ECDSA keypair successful
[2020-04-26T08:19:15Z DEBUG substratee_worker_enclave::attestation] pubkey X is bf6df80bec01cc4d89a56de8146cf5040c4da8e9ff4081edced05b40d1b1a7d9
[2020-04-26T08:19:15Z DEBUG substratee_worker_enclave::attestation] pubkey Y is c2478c87710ac25aaed30d125c63edb3beba25b5564a5f222fdc09298bad2a67
...
2020-04-26T08:19:17Z DEBUG substratee_worker_enclave::attestation] payload 93cbb905e945dd817dfa86ff52e1261e7ff6956cf8b76e05e936090aa295138c
[2020-04-26T08:19:17Z DEBUG substratee_worker_enclave::attestation] signature: 3fadd5de3212cd7b1bdeaf4d8cec4852c38ac5876abcca66737477fa74d22025a2656e6e4873f2afeeb61d79e15cf749f98bbc6eada7a8c5c29d968ed42d6aa3
```
Now I'm trying to verify the signature with [ring](https://github.com/briansmith/ring). But the pubkey seems not to be on the curve (I also tried reversing the bytes in case it was little-endian). Also boringSLL thinks it's not on the curve.

From these lines
https://github.com/intel/linux-sgx/blob/4589daddd58bec7367a6a9de3fe301e6de17671a/sdk/tlibcrypto/sgxssl/sgx_ecc256.cpp#L57

I would conclude that ecc_handle is using the prime256v1 curve, equivalent to the NIST P-256 / secp256r1 that ring refers to with `ECDSA_P256_SHA256_FIXED`

So I'd concatenate X and Y and prefix it with `0x04` to pass it to ring:
```
04 bf6df80bec01cc4d89a56de8146cf5040c4da8e9ff4081edced05b40d1b1a7d9 c2478c87710ac25aaed30d125c63edb3beba25b5564a5f222fdc09298bad2a67
```

So why is the pubkey not on the curve?