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The growth of binaries over time and the inclusion of new features filled the available boot partition space, so that the kernel+initrd almost couldn't fit twice anymore as required for updates. We employed workarounds such as wrapper scripts for ignition, afterburn and other binaries so that they are loaded from /usr. However, this was still not enough and we would have to do the same for (network) kernel modules and firmware. To avoid making this ever more complex we can use a dedicated initrd focused on loading the full initrd from /usr and then this full initrd can use dracut as before and even drop all the workarounds we accumulated. Generate a minimal initrd to use instead of the full bootengine initrd. The bootengine initrd gets stored as squashfs on /usr. The minimal initrd still includes the early_cpio for amd64 microcode updates. We have a fixed list of modules or module directories to include, only focused on loading /usr and any emergency console interaction. This requires also checking for module dependencies to copy over. The busybox, veritysetup, and kmod binaries are needed and get their required libraries resolved and copied over. They are not static and use shared libraries which should be ok for now. The resulting vmlinuz file is 27 MB for amd64, down from ~60 MB, so we have enough room to include more kernel modules and so on for the next years while we also grow the boot partition and wait for users to redeploy until we can rely on a larger boot partition and eventually drop the minimal initrd again. Pulls in https://github.com/flatcar/bootengine/pull/110 for the minimal initrd script and https://github.com/flatcar/seismograph/pull/12 for making the device mapper discovery for the "rootdev" command more reliable. This also requied a backport of a kernel patch from 2017 that exposes the PARTUUID in the /sys uevent file. Co-authored-by: James Le Cuirot <jlecuirot@microsoft.com> Signed-off-by: Kai Lueke <kailuke@microsoft.com>