Detect first boot based on the existence of a coreos/first_boot file
in the EFI partition, and set "coreos.first_boot=detected" command line
argument when found. We use "detected" rather than "1" so the initramfs
knows that it should mount the ESP and delete the file. This lets us
defer clearing the first-boot flag until Ignition has run successfully,
without having to change the disk GUID after filesystems are mounted.
Continue detecting the first-boot disk GUID and adding the command-line
argument to randomize it, since we still want unique disk GUIDs
regardless of Ignition.
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