Having been bitten by a limited /boot partition running out of space when dist-upgrading to 17.10 - it is helpful to remove dated and unused linux-header image and extra packages, either before the upgrade, or periodically, as these all get re-built onto /boot as part of the disposal of old packages during the upgrade.
for upg in 20 22 24 26 28
do
sudo dpkg -r --purge linux-headers-4.10.0-${upg} linux-headers-4.10.0-${upg}-generic linux-image-4.10.0-${upg}-generic linux-image-extra-4.10.0-${upg}-generic
done
I’ve heard that fixing automatic removal of old kernels would be properly tackled in the near future (doesn’t work in some corner cases); unfortunately I can’t remember a source.