Finish merged-usr process for generic images, move SDK over to it too

[krnowak]

Current situation:

• SDK:
  • /bin, /sbin, /lib, /lib64, /usr/bin, /usr/sbin, /usr/lib, /usr/lib64 are all directories
• generic:
  • /usr/bin, /usr/sbin, /usr/lib, /usr/lib64 are all directories
  • /bin, /sbin, /lib, /lib64 are symlinks to their counterparts in /usr.

New situation for both (essentially known as merged-usr in Gentoo):

• /usr/bin, /usr/lib, /usr/lib64 are all directories
• /usr/sbin is a symlink to /usr/bin
• /bin, /sbin, /lib, /lib64 are symlinks to their counterparts in /usr.

CI: http://jenkins.infra.kinvolk.io:8080/job/container/job/sdk/1516/cldsv/

• [ ] Changelog entries added in the respective changelog/ directory (user-facing change, bug fix, security fix, update)
• [ ] Inspected CI output for image differences: /boot and /usr size, packages, list files for any missing binaries, kernel modules, config files, kernel modules, etc.

[krnowak]

Filed a fix for Gentoo: https://github.com/gentoo/gentoo/pull/37397

[github-actions[bot]]

Build action triggered: https://github.com/flatcar/scripts/actions/runs/9758685125