Feature request - amazon-chroot builder - post_provision steps and custom chroot_path

_This issue was originally opened by @anderssvanqvist as hashicorp/packer#4824. It was migrated here as a result of the [Packer plugin split](https://github.com/hashicorp/packer/issues/8610#issuecomment-770034737). The original body of the issue is below._

I am using the amazon-chroot builder to do an image installation of VyOS which mounts the actual disk at /boot and has the root device as an overlay on a squashfs and a version subdirectory under boot for writes.

The amazon-chroot builder tightly couples the ebs device and the chroot path which does not work in this case. It also does not have a post_provision step which means that I can't do any cleanup of the various mounts. Right now I'm doing everything in the post_mount_commands step which means manually scripting the mounts and copy files.

My request is this:

* Implement a post_provision_commands step, that runs after provision but before unmount.

* Add a chroot_path variable to support use cases like writable squashfs overlays.

Tia,
Anders