bootc should produce useful output for interaction with external scripts

[jmpolom]

As suggested in #421 (comment) and #440 more complex cases of installing a system with bootc to disk need support from an external "installer".

The simplest case here is probably some kind of installation script shipped with the container. There are tasks this script might do like inject some configuration files after bootc deploys the ostree. Injecting files into the ostree deployment is currently not supported by bootc and effectively a broken thing (#531) because finding the deployment path isn't straightforward or trivial. bootc does not output this path during installation and the current installation CLI output are pretty much just status messages.

The ostree CLI tools do not seem to have a command that cleanly produces a path to the deployment so one resorts to hacky things like stringing together several shell commands to get to the path. This is unfortunate because in several places bootc is actually already getting this path information during installation via the ostree rust crates but isn't exposing it in any useful way to the user. This should be corrected to improve usability.

The output from bootc status should include path(s) to the deployment directories (bootc status produces the deploy hash on a booted system) The output from the installation sub-command should be configurable to produce some useful output cleanly. This output should include among other things a (relative?) path to the deployment under /ostree in the physical root. bootc status could also be modified to produce some more useful output on a non-booted system if you could point it to the sysroot (like the existing ostree CLI tools).