Write build spec to fd.

def WriteBuildYaml(self, fd, build_timestamp=True):
    """Write build spec to fd."""
    output = {
        "Client.build_environment":
            rdf_client.Uname.FromCurrentSystem().signature(),
        "Template.build_type":
            config.CONFIG.Get("ClientBuilder.build_type", context=self.context),
        "Template.version_major":
            config.CONFIG.Get("Source.version_major", context=self.context),
        "Template.version_minor":
            config.CONFIG.Get("Source.version_minor", context=self.context),
        "Template.version_revision":
            config.CONFIG.Get("Source.version_revision", context=self.context),
        "Template.version_release":
            config.CONFIG.Get("Source.version_release", context=self.context),
        "Template.arch":
            config.CONFIG.Get("Client.arch", context=self.context)
    }

    if build_timestamp:
      output["Client.build_time"] = rdfvalue.RDFDatetime.Now()
    else:
      self.REQUIRED_BUILD_YAML_KEYS.remove("Client.build_time")

    for key, value in iteritems(output):
      output[key] = str(value)

    output["Template.build_context"] = self.context

    output_keys = set(iterkeys(output))
    if output_keys != self.REQUIRED_BUILD_YAML_KEYS:
      raise RuntimeError("Bad build.yaml: expected %s, got %s" %
                         (self.REQUIRED_BUILD_YAML_KEYS, output_keys))
    fd.write(yaml.Dump(output).encode("utf-8"))