I've verified this on xenial using the steps provided in the test case above.
$ uname -r
4.8.0-28-generic
$ dpkg -S /boot/vmlinuz-$(uname -r)
linux-image-4.8.0-28-generic: /boot/vmlinuz-4.8.0-28-generic
$ dpkg-query --show overlayroot
overlayroot 0.27ubuntu1.3
I've verified this on xenial using the steps provided in the test case above. $(uname -r) 4.8.0-28- generic: /boot/vmlinuz- 4.8.0-28- generic
$ uname -r
4.8.0-28-generic
$ dpkg -S /boot/vmlinuz-
linux-image-
$ dpkg-query --show overlayroot
overlayroot 0.27ubuntu1.3