mkinitcpio: Produce reproducible initramfs images

Created by: esotericnonsense

We achieve this by stripping timestamps from within the filesystem, and by using a pipeline to strip inodes from the cpio archive.

It functions for at least the 'gzip', 'xz', 'bzip2', 'lz4' and 'cat' compressors. The 'lzop' compressor embeds a runtime timestamp.

Motivation: https://reproducible-builds.org

Signed-off-by: Daniel Edgecumbe git@esotericnonsense.com