Fix logic when enabling buildkit

Signed-off-by: Tibor Vass <tibor@docker.com>

api/server/router/build/build_routes.go

@@ -231,7 +231,7 @@ func (br *buildRouter) postBuild(ctx context.Context, w http.ResponseWriter, r *
 	}
 
 	// check if the builder feature has been enabled from daemon as well.
-	if buildOptions.Version == types.BuilderBuildKit && br.builderVersion != types.BuilderBuildKit {
+	if buildOptions.Version == types.BuilderBuildKit && br.builderVersion != "" && br.builderVersion != types.BuilderBuildKit {
 		return errdefs.InvalidParameter(errors.New("buildkit is not enabled on daemon"))
 	}
 

daemon/config/config.go

@@ -54,6 +54,7 @@ var flatOptions = map[string]bool{
 	"log-opts":           true,
 	"runtimes":           true,
 	"default-ulimits":    true,
+	"features":           true,
 }
 
 // skipValidateOptions contains configuration keys