Dropping the rootfs requirement and restoring arbitrary bundle content

Dropping the rootfs requirement and restoring arbitrary bundle content

Tag this thread, which allows for bundles that don't pivot-root, and
allows bundle authors to distribute additional content.  However, the
root filesystem requirement is enshrined in the adopted charter [1,2],
so the 'charter' tag flags it for TOB action once we get a TOB.

[1]: https://www.opencontainers.org/governance
[2]: Message-ID: <20150826195447.GX21585@odin.tremily.us>