Actualizar Raspberry Pi 3 de Ubuntu Mate 16.04.5 a Ubuntu Mate 18.04

Al intentar actualizar la Raspberry Pi de ubuntu mate 16.04.5 a ubuntu 18.04.1, al ejecutar sudo do-release-upgrade, recibimos un mensaje indicándonos no existe bastante espacio libre en /boot.

Ejemplo :

The upgrade has aborted. The upgrade needs a total of 49.5 M free 
space on disk '/boot'. Please free at least an additional 5,374 k of 
disk space on '/boot'. You can remove old kernels using 'sudo apt 
autoremove' and you could also set COMPRESS=xz in 
/etc/initramfs-tools/initramfs.conf to reduce the size of your 
initramfs. 

root@rpi3b2:~$ ls -la /boot

Solución :

Mover los ficheros kernel.img y start_x.elf a un lugar seguro.

sudo mv /boot/kernel.img /

sudo mv /boot/start_x.elf /

Ejecutar de nuevo : sudo do-release-upgrade