Ubuntu /boot full

First, Ubuntu 16.04 LTS has reached its end-of-life in last month, so you need to update to latest Ubuntu ASAP, otherwise you are running outdated software and exposing your system to all kinds of compromises.

You can try to remove the old kernel packages as follows:

Get a list of all kernel installed kernel packages:

dpkg -l | grep linux-image

Purge each package from the system:

apt purge linux-image-4.4.0-176-generic

where you run the command for each but two latest kernel packages you have installed.

After purging old packages, you can try running apt-get -f install to complete installation of missing dependencies.