Try to do a release upgrade I ran into the issue of /boot not having enough free space. Here is how to resolve that.
Check which kernel you are actively using. DO NOT REMOVE THIS ONE!
uname -r
View all kernels installed on your system.
dpkg --list | grep linux-image
Remove the kernels not in use
apt-get purge linux-image-x.x.x.x-generic
Update grub
update-grub2
Reboot