We'll need a mechanism that will allow a cluster admin to opt in to on-cluster builds for MachineConfig layering. This could be detecting for the presence of a certain object, the presence of a certain ConfigMap key, etc. Regardless of the opt-in implementation, the following things should happen whenever the opt-in mechanism is activated:

• The MCO starts the Machine OS Builder binary in its own separate pod. Upon startup, it should get the currently active MachineConfig for each MachineConfigPool, build an OS image for all of them, then roll them out.
• The MCD is aware that it should only apply the OS image and any other non-MachineConfig content, such as certificates. It should not write any systemd units or files in this mode.
• If the opt-in mechanism is then set to false (i.e., the cluster admin wants to roll back to non-layered MachineConfigs), then the Machine OS Builder pod should be shut down, all nodes should revert back to the stock OS, and the MCD should apply MachineConfigs as it currently does today.

Done When:

• An opt-in mechanism is selected and implemented.
• The MCO is aware of the opt-in mechanism and starts the Machine OS Builder binary.
• The MCD is aware of the opt-in mechanism and behaves accordingly.