make-initrd: create reproducible initrds

To achieve reproducible results, `cpio` archive members are added in
sorted order and inodes renumbered.

The `cpio-clean.pl` script is made obsolete by setting mtimes via
`touch` & using `cpio --reproducible`.  Suggested by @dezgeg in
https://github.com/NixOS/nixpkgs/pull/21273#issuecomment-268116605.

Note that using `--reproducible` means that initial ramdisk creation now
requires at least `cpio` version 2.12 (released in 2015).