Overlayfs is part of the kernel, while unionfs needs FUSE. This also reduces
the size of the initrd by ca. 4.3% (487K).
* gnu/build/linux-boot.scm (mount-root-file-system): Remove optional parameter
"unionfs"; mount using overlayfs instead of unionfs; new directory layout
requied by overlayfs; update documentation.
[mark-as-not-killable]: Remove now unused function
* gnu/system/linux-initrd.scm (file-system-packages): Remove now unused
packages "unionfs-fuse/static" and thus unused related 'if'.
(linux-modules): Replace "fuse" by "overlay".
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