Add eclass/usr-ldscript.eclass to package automation

[krnowak]

CI: http://jenkins.infra.kinvolk.io:8080/job/container/job/sdk/1781/cldsv/

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It couldn't be added before to automation, because the eclass in Gentoo would introduce have unwanted side-effects into our built images. But the Gentoo version of the eclass is essentially a no-op when USE=split-usr is disabled. We have recently moved to use Gentoo profiles that disable USE=split-usr altogether, and with this move, we can now safely put the eclass into automation.

This marks the point where the entirety of the portage-stable is under automation.

Closes https://github.com/flatcar/Flatcar/issues/645. Closes https://github.com/flatcar/Flatcar/issues/646.

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• [ ] Inspected CI output for image differences: /boot and /usr size, packages, list files for any missing binaries, kernel modules, config files, kernel modules, etc.