free disc space in /boot by removing old kernels

Once in a while Ubuntu can't update, because there is not enough free space in /boot.

To free space, old, unused kernels have to be removed. This can automatically be done with: 

sudo apt-get purge $(dpkg -l linux-{image,headers}-"[0-9]*" | awk '/ii/{print $2}' | grep -ve "$(uname -r | sed -r 's/-[a-z]+//')")

From:

https://askubuntu.com/questions/89710/how-do-i-free-up-more-space-in-boot