prepare-root: Switch to a tmpfs for transient root

prepare-root: Switch to a tmpfs for transient root

We're debating this over in https://github.com/CentOS/centos-bootc-dev/pull/27
and I have come to the conclusion that having changes to `/`
persist across reboot by default was a bad idea.

- It conflicts with any kind of secure boot scenario
- Having things only go away on upgrades is in some ways even *more* surprising
- The term `transient` implies this

There may be a use case in the future for having something like `root.transient = persistent`,
but this is just a better default.

Signed-off-by: Colin Walters <walters@verbum.org>