Add optional read-write tmpfs overlay to rootfs

[brl]

It would be nice to have an easy way to enable a writable tmpfs overlay mounted on the rootfs. This would would simply development workflow, but also it would be a good tool to detect components that are silently failing to write to the read-only filesystem. The system could be run for a while with this overlay enabled and then the upperdir can be examined for unexpected files. I think it's highly possible that some components will just fail to write files without generating any log messages and that these failed writes could result in subtle breakage that would be very hard to otherwise track down.