rpm-ostree ex persistoverlay

[cgwalters]

We could probably fairly easily generalize support for one or more "persistent overlay" trees from our package layering support. Something like this:

( build podman from git master in a container, copy the binary to `/var/tmp/podman/usr/bin/podman` )
# rpm-ostree ex persistoverlay create podman-from-git /var/tmp/podman
# rpm-ostree status
State: idle; auto updates disabled
Deployments:
  ostree://fedora-atomic:fedora/28/x86_64/workstation
                   Version: 28.20180521.0 (2018-05-21 13:54:32)
                BaseCommit: af6c1805fdd4740fbabec178e7b3ba14cbf0a07c1f66159de18f9b9962769fbe
               PersistentOverlays: podman-from-git
# ostree ls rpmostree/persistent-overlays/podman-from-git /usr/bin/podman
0755 ... /usr/bin/podman
# rpm-ostree ex persistoverlay checkout podman-from-git /var/tmp/podman
(copy new data into /var/tmp/podman)
# rpm-ostree ex persistoverlay update podman-from-git /var/tmp/podman
# rpm-ostree ex persistoverlay remove podman-from-git 

etc. The use case here is for things that aren't RPMs - it's like ostree admin unlock --hotfix but actually persistent across upgrades.

One thing people might wonder; what about doing RPMs this way? Something like doing rpm2cpio and then committing that as a persistlayer? The problem with that is you get conflicts on the RPM database - that's one of the things rpm-ostree pkg layering knows how to handle.

[cgwalters]

Marking this as a dup of #2326 (i'd forgotten about this one when I wrote that), and 2326 is more comprehensive.