Propose a mechanism to enable sysext builds in the OS image build process

[t-lo]

The OS image we currently produce is unsuitable as a base for building Flatcar specific sysext images: it lacks package information and portage configuration. During the image build process our low-level build automation removes this information from the OS image to keep it lean.

In order to build sysext images suitable for running on a specific OS release, the image build needs to be extended to produce artifacts retaining this information.

Part of the work is already in progress here: flatcar/scripts#858

This issue builds on that work with the aim to make it more generally usable for all future Flatcar / version specific sysext builds.

[t-lo]

@krishjainx wanted to look into this.

[krishjainx]

I think we can already close this since it's merged? @t-lo

[jepio]

Not until the build process produces docker/containerd sysexts that work.

[t-lo]

Good point, @jepio .
PR was merged and will be in Alpha-3654 (flatcar/scripts#920).
Also, we're using build_sysext now to build the OEM sysexts (flatcar/scripts#969).

[pothos]

The script is now used for both the OEMs as well as the inbuilt Docker+containerd sysext images.