Comment 4 for bug 1615021

Examining the initrd shows that readlink is provided as /usr/bin/readlink -> /bin/busybox, not as /bin/readlink where systemd expects it (and where it's shipped on an installed system). This is a bug in debian-installer's construction of that image - though gee it would be nice if systemd didn't require hard-coded paths to everything.

There's no guarantee that fixing the bug that's causing this error message will fix the underlying problem preventing your boot, but it will at least fix the message spam.