If you constantly run out of space in the boot partition, you might need to run the following command to cleanup the boot partition.
First, list all installed kernels
dpkg -l linux-image-* | grep ii
Second, delete the chosen old kernel
apt-get purge linux-image-4.4.0-XXXX*
Other command worth noting:
apt-get --purge autoremove
You can also remove the old kernel manually. I suggest leaving the older one and the latest.
Delete the kernels in between.