Sign channel IDs with ECDSA_do_sign.

Rather than parse with d2i_ECDSA_SIG and reserialize, this is cleaner.
It's also clearer that i2d_PublicKey isn't being used for DER.

Change-Id: Iac57fb6badd1dfed1e66984e95a31f609b1538a4
Reviewed-on: https://boringssl-review.googlesource.com/5263
Reviewed-by: Adam Langley <agl@google.com>