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).

pkgs/build-support/kernel/make-initrd.sh

@@ -39,7 +39,8 @@ mkdir -p $out
 for PREP in $prepend; do
   cat $PREP >> $out/initrd
 done
-(cd root && find * -print0 | cpio -o -H newc -R 0:0 --null | perl $cpioClean | $compressor >> $out/initrd)
+(cd root && find * -print0 | xargs -0r touch -h -d '@1')
+(cd root && find * -print0 | sort -z | cpio -o -H newc -R +0:+0 --reproducible --null | $compressor >> $out/initrd)
 
 if [ -n "$makeUInitrd" ]; then
     mv $out/initrd $out/initrd.gz