Support booting ISO completely from RAM

[dustymabe]

We've always supported "live" uses cases for our ISO image but right now our ISO image boots and mounts the rootfs from the ISO media. This means that part of the booted system still references something that isn't in memory. I think the assumption at the time we were building this is that the ISO would continue to be available during the duration of that system boot, but there have been several use cases presented recently where the user wants to boot an ISO completely from RAM.

1. A case where the user wants to actually "install" directly to the same device that was used to boot the live ISO image (i.e. the ISO was written to a block device and then booted from).
2. A case where the user wants to run a "live" system (diskless) from a USB media, but then remove that USB media once the system is booted.

We should consider supporting these use cases.

[dustymabe]

I created this tracker issue a bit retroactively for completeness. The work for this was done in coreos/fedora-coreos-config#2544 in which support for a coreos.liveiso.fromram kernel argument was added precisely for these use cases.

[dustymabe]

docs: coreos/fedora-coreos-docs#577

[dustymabe]

The fix for this went into next stream release 38.20230819.1.0. Please try out the new release and report issues.

[dustymabe]

The fix for this went into testing stream release 38.20230819.2.0. Please try out the new release and report issues.

[dustymabe]

The fix for this went into stable stream release 38.20230819.3.0.