busybox-initramfs binary package in Ubuntu Xenial amd64

 BusyBox combines tiny versions of many common UNIX utilities into a single
 small executable. It provides minimalist replacements for the most common
 utilities you would usually find on your desktop system (i.e., ls, cp, mv,
 mount, tar, etc.). The utilities in BusyBox generally have fewer options than
 their full-featured GNU cousins; however, the options that are included
 provide the expected functionality and behave very much like their GNU
 counterparts.
 .
 busybox-initramfs provides a simple stand alone shell that provides
 only the basic utilities needed for the initramfs.

Source package