Free space from boot

See what all linux images/kernels are present in your system

dpkg -l linux-image-\* | grep ^ii

Now see what all linux kernels you can remove

kernelver=$(uname -r | sed -r 's/-[a-z]+//')

mongoexample$ dpkg -l linux-{image,headers}-"[0-9]*" | awk '/ii/{print $2}' | grep -ve $kernelver

Now remove the linux kernels

sudo apt-get purge linux-image-4.4.0-70-generic