rootfs: fix compilation error

Signed-off-by: Akihiro Suda <suda.akihiro@lab.ntt.co.jp>

vendor/github.com/opencontainers/image-spec/identity/helpers.go

@@ -0,0 +1,40 @@
+// Copyright 2016 The Linux Foundation
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package identity
+
+import (
+	_ "crypto/sha256" // side-effect to install impls, sha256
+	_ "crypto/sha512" // side-effect to install impls, sha384/sh512
+
+	"io"
+
+	digest "github.com/opencontainers/go-digest"
+)
+
+// FromReader consumes the content of rd until io.EOF, returning canonical
+// digest.
+func FromReader(rd io.Reader) (digest.Digest, error) {
+	return digest.Canonical.FromReader(rd)
+}
+
+// FromBytes digests the input and returns a Digest.
+func FromBytes(p []byte) digest.Digest {
+	return digest.Canonical.FromBytes(p)
+}
+
+// FromString digests the input and returns a Digest.
+func FromString(s string) digest.Digest {
+	return digest.Canonical.FromString(s)
+}