The only things needed to successfully boot and mount a root filesystem are:
/bin/sh, /dev/console and 'init', located either in /sbin/init, /bin/init or /etc/init. sh and/or init can even be a link to something else -most likely a linuxrc file. So, you can create a statically-compiled binary called linuxrc, create links from /bin/sh and /sbin/init to it. That gives you a (possibly) complete running system with any purpose -or none as you choose. One file, one device file and two links. Put that into an initramfs and compile it into the kernel and you have a 'complete' system from a single file.