``Not enough free disk space'' message when trying a do-release-upgrade from ubuntu 16.04 to 18.04 on a RPi 3 B

Question

Upgrade response:

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
total 20508
drwxr-xr-x  4 root root   16384 Dec 31  1969 .
drwxr-xr-x 22 root root    4096 Jan 24  2018 ..
-rwxr-xr-x  1 root root   14010 Dec 19  2016 bcm2708-rpi-b.dtb
-rwxr-xr-x  1 root root   14273 Dec 19  2016 bcm2708-rpi-b-plus.dtb
-rwxr-xr-x  1 root root   13964 Dec 19  2016 bcm2708-rpi-cm.dtb
-rwxr-xr-x  1 root root   15356 Dec 19  2016 bcm2709-rpi-2-b.dtb
-rwxr-xr-x  1 root root   15992 Dec 19  2016 bcm2710-rpi-3-b.dtb
-rwxr-xr-x  1 root root   15350 Dec 19  2016 bcm2710-rpi-cm3.dtb
-rwxr-xr-x  1 root root   17932 Dec 19  2016 bootcode.bin
-rwxr-xr-x  1 root root     181 Dec 31  1979 cmdline.txt
-rwxr-xr-x  1 root root   36771 Oct 21 12:49 config.txt
-rwxr-xr-x  1 root root   18693 Dec 19  2016 COPYING.linux
-rwxr-xr-x  1 root root    2535 Dec 19  2016 fixup_cd.dat
-rwxr-xr-x  1 root root    6622 Dec 19  2016 fixup.dat
-rwxr-xr-x  1 root root    9753 Dec 19  2016 fixup_db.dat
-rwxr-xr-x  1 root root    9753 Dec 19  2016 fixup_x.dat
drwxr-xr-x  2 root root    8192 Dec 14  2017 grub
-rwxr-xr-x  1 root root 4231256 Dec 19  2016 kernel7.img
-rwxr-xr-x  1 root root 4130008 Dec 19  2016 kernel.img
-rwxr-xr-x  1 root root    1494 Dec 19  2016 LICENCE.broadcom
drwxr-xr-x  2 root root    8192 Feb 11  2017 overlays
-rwxr-xr-x  1 root root  634532 Dec 19  2016 start_cd.elf
-rwxr-xr-x  1 root root 4956996 Dec 19  2016 start_db.elf
-rwxr-xr-x  1 root root 2823396 Dec 19  2016 start.elf
-rwxr-xr-x  1 root root 3906116 Dec 19  2016 start_x.elf

What can be done to recover the 5.374Mb space needed in /boot?

Thanks in advance.

Answer 1

You have not specified exactly WHAT Ubuntu - this could be one of several.

1. The problem with the graphics update on 16.04 is well known (although the cause may not/or no one cares enough to fix it).

   You can update from the command line with apt update && apt upgrade.

2. If the process did not fail you would have destroyed your OS. do-release-upgrade is for normal Ubuntu installations and attempts to install the boot-loader, NOT for the Pi which has a custom boot process.
3. There is NO working/supported Ubuntu 18.04 for the Pi. See https://raspberrypi.stackexchange.com/a/90899/8697
4. You do not appear to have updated your system in the last 2 years. Ubuntu is now at 16.04.5 - including many fixes. Updating would be normal before attempting an upgrade (which is unlikely to be available).