Pre-apply Ignition kargs at installation time as an optimization

[jlebon]

Feature Request

Desired Feature

If a target Ignition config is provided to the installer, and it is "flattened" (i.e. no replacements or merges), then coreos-installer could learn to pre-apply any kernel arguments in the Ignition config. This will avoid a system reboot, which can be costly on some bare metal systems. And in the case of console kargs, the boot we eliminate is confusing because there's no feedback available.

Related: coreos/fedora-coreos-tracker#1116

Other Information

This is easier to do now that coreos-installer knows about the Ignition schema from the iso/pxe customize work.