Step: unpack-rootfs
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Unpack a tarball of the root filesystem to the image, and set the
`rootfs_unpacked` condition to true. If the tarball doesn't exist, do
nothing and leave the `rootfs_unpacked` condition to false.

Step keys:

* `unpack-rootfs` &mdash; REQUIRED; tag for the root filesystem.

Example (in the .vmdb file):

    - unpack-rootfs: root
