Add boringssl to CI

* BoringSSL does not use the bindgen feature, since it generates the
  crate separately.
* BoringSSL shouldn't run the systest tests (maintained out of tree)
  or the openssl-errors tests (BoringSSL doesn't support external
  errors)
* Adds both master and a known-working revision to CI. The master build
  should be made non-blocking.