build: Use overlayfs instead of unionfs.

build: Use overlayfs instead of unionfs.

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".