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