Externalize TarBits Extract function

This patch makes it possible to stage bits as a distinct step in the
TarBits Build process by invoking
pkg/build/kube.TarBitsExtract(kubeRoot, extractPath).

pkg/build/kube/tarbits.go

@@ -62,6 +62,20 @@ func init() {
 	RegisterNamedBits("tar", NewTarBits)
 }
 
+// TarBitsExtract extracts bits from a given kubeRoot to the given extractPath
+// Please see NewTarBits for information on the valid kubeRoot values.
+func TarBitsExtract(kubeRoot, extractPath string) error {
+	bits, err := NewTarBits(kubeRoot)
+	if err != nil {
+		return err
+	}
+	tarBits := bits.(*TarBits)
+	if _, err := tarBits.extract(tarBits.kubeRoot, extractPath); err != nil {
+		return err
+	}
+	return nil
+}
+
 // NewTarBits returns a new Bits backed by a version file, the release image
 // archives and the kubeadm, kubectl, and kubelet binaries. The given kubeRoot
 // may be defined three ways: