command/build: "end" sentinel in artifact machine-readable output

CHANGELOG.md

@@ -15,6 +15,8 @@ IMPROVEMENTS:
 * core: Output message when Ctrl-C received that we're cleaning up. [GH-338]
 * builder/amazon: Tagging now works with all amazon builder types.
 * command/build: Machine-readable output now contains build errors, if any.
+* command/build: An "end" sentinel is outputted in machine-readable output
+  for artifact listing so it is easier to know when it is over.
 
 BUG FIXES:
 

command/build/command.go

@@ -235,6 +235,7 @@ func (c Command) Run(env packer.Environment, args []string) int {
 					ui.Machine("artifact", iStr, "nil")
 				}
 
+				ui.Machine("artifact", iStr, "end")
 				env.Ui().Say(message.String())
 			}
 		}

website/source/docs/machine-readable/command-build.html.markdown

@@ -61,6 +61,15 @@ of `packer build`.
 		</p>
 	</dd>
 
+	<dt>artifact subtype: end (0)</dt>
+	<dd>
+		<p>
+		The last machine-readable output line outputted for an artifact.
+		This is a sentinel value so you know that no more data related to
+		the targetted artifact will be outputted.
+		</p>
+	</dd>
+
 	<dt>artifact subtype: file (2)</dt>
 	<dd>
 		<p>