Improve observability of ISO/PXE customizations

[bgilbert]

Feature Request

Desired Feature

At the moment there's no way to inspect a customized ISO image or initramfs to get a concise summary of what's been customized. For an ISO image, a user can only iso reset and start over; while for a PXE image, they can't even reset, and need to rebuild from a pristine image.

Consider adding e.g. iso describe and pxe describe commands that summarize the customizations in an image.

Similarly, customized images could log a summary of the customizations to the journal during the live boot, for post-boot diagnosis. It may also make sense for the (first) installed boot to summarize the customizations as well, or the relevant subset of them.

Other Information

We probably don't want to try to programmatically decompile the various configs in an image. However, we could generate a human-readable summary during customization, and embed it as a file in the generated Ignition config(s) along with a systemd service that dumps it to the journal. The describe commands could just extract and display that file, with fallbacks for older customized images and those customized with the lower-level iso ignition/iso network/etc. commands.

We'd also need to figure out what information would be useful to include in the human-readable summary.

Suggested by @control-d (describe commands) and @dustymabe (journal entries).