Merge pull request #8659 from hashicorp/update_amznchroot_docs

Add template engine variables doc for amazon-chroot

website/source/docs/builders/amazon-chroot.html.md.erb

@@ -290,3 +290,14 @@ variables are available:
 -   `SourceAMIOwner` - The source AMI owner ID.
 -   `SourceAMIOwnerName` - The source AMI owner alias/name (for example `amazon`).
 -   `SourceAMITags` - The source AMI Tags, as a `map[string]string` object.
+
+## Build function template engine variables
+
+For the build function of [template engine](/docs/templates/engine.html), the following
+variables are available:
+
+-   `SourceAMIName` - The source AMI Name (for example
+    `ubuntu/images/ebs-ssd/ubuntu-xenial-16.04-amd64-server-20180306`) used to
+    build the AMI.
+-   `Device` - Root device path.
+-   `MountPath` - Device mounting path.