simplify BlockDevice prepares

builder/amazon/chroot/block_device.go

@@ -44,10 +44,11 @@ func (blockDevice BlockDevice) BuildEC2BlockDeviceMapping() *ec2.BlockDeviceMapp
 }
 
 func (b *BlockDevice) Prepare(ctx *interpolate.Context) error {
-	if b.DeviceName == "" {
+	err := b.BlockDevice.Prepare(ctx)
-		return fmt.Errorf("The `device_name` must be specified " +
+	if err != nil {
-			"for every device in the block device mapping.")
+		return err
 	}
+
 	// Warn that encrypted must be true when setting kms_key_id
 	if b.KmsKeyId != "" && b.Encrypted != nil && *b.Encrypted == false {
 		return fmt.Errorf("The device %v, must also have `encrypted: "+

builder/amazon/ebssurrogate/block_device.go

@@ -3,7 +3,6 @@
 package ebssurrogate
 
 import (
-	"fmt"
 	"strings"
 
 	"github.com/aws/aws-sdk-go/aws"
@@ -86,15 +85,6 @@ func (blockDevice BlockDevice) BuildEC2BlockDeviceMapping() *ec2.BlockDeviceMapp
 	return mapping
 }
 
-func (b *BlockDevice) Prepare(ctx *interpolate.Context) error {
-	if b.DeviceName == "" {
-		return fmt.Errorf("The `device_name` must be specified " +
-			"for every device in the block device mapping.")
-	}
-
-	return nil
-}
-
 func (bds BlockDevices) Prepare(ctx *interpolate.Context) (errs []error) {
 	for _, block := range bds {
 		if err := block.Prepare(ctx); err != nil {