Add support for FreeBSD to azure chroot builder

[backerman]

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Description

The Azure chroot builder currently only supports Linux, but I don't see a reason it wouldn't be portable to other Unix-like systems. Since I need FreeBSD support, I'm starting with that. (And I'm working on a PR at this very moment.)

Use Case(s)

Building FreeBSD images with the Azure chroot builder.

Potential configuration

{
  "variables": {
  },
  "builders": [
    {
      "type": "azure-chroot",
      "source": "thefreebsdfoundation:freebsd-12_1:12_1-release:latest",
      "image_resource_id": "/subscriptions/{{vm `subscription_id`}}/resourceGroups/{{vm `resource_group`}}/providers/Microsoft.Compute/images/freebsd-{{timestamp}}",
      "os_disk_size_gb": 64,
      "os_disk_storage_account_type": "Premium_LRS",
      "mount_partition": 2,
      "chroot_mounts": [
        ["devfs", "devfs", "/dev"],
        ["procfs", "procfs", "/proc"]
      ]
    }
  ],
  "provisioners": [
    {
      "inline": [
        "touch /test"
      ],
      "inline_shebang": "/bin/sh -x",
      "type": "shell"
    }
  ]
}

Potential References

[ghost]

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