Comment 10 for bug 1421117

initramfs-tools already depends busybox-initramfs. What's missing is the hook to symlink bin/chroot to busybox in the initrd.
Supporting /sbin/init being a symlink was not fixed for all cases in bug #1351295. There was only a readlink symlink added but the chroot call in the following line was missed. By coincidence the busybox-static hook installed a chroot symlink so it worked for most cases (busybox-static is a dependency of ubuntu-standard).