What to do if /boot is too small

# create copy of /boot on root partition
mkdir /boot2
rsync -a /boot/ /boot2/
umount /boot
rmdir /boot
mv /boot2 /boot

# remove /boot entry from /etc/fstab
$EDITOR /etc/fstab

# install linux-image-amd64 to have all initrd tools installed
apt-get install linux-image-amd64

# create all initrd if some are missing (look at installed stuff under /boot)
# update-initramfs -k VERSION -c
update-initramfs -k all -u

# update/re-install grub 
update-grub
grub-install '(hd0)'

# check /boot/grub/grub.cfg