fsck: Don't add fsck binary for OverlayFS

Created by: M3t0r

When using OverlayFS on the root mount, the fsck hook will attempt to add the corresponding fsck binary, which doesn't exist.

  -> Running build hook: [fsck]
==> ERROR: file not found: `fsck.overlay'

My mount output on the installation where I encountered this error:

none on / type overlay (rw,relatime,lowerdir=base/,upperdir=overlay/overlay-upper/,workdir=overlay/overlay-work/)