Can't update Ubuntu Base using software update

When I try to update Ubuntu base using the software updater tool, I get told that I don't have enough room in /boot.

The upgrade needs a total of 48.2 M free space on disk '/boot'. Please free at least an additional 4,088 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.

I have run 'sudo apt autoremove' and made the change to initramfs.conf, but still get the error. I have 57.6 GB of free storage left on my SD card so that shouldn't be the issue. I looked at the properties for my /boot directory is and it is using 21.1 MB and has 44.1MB of free space. I'd appreciate any advice you can give me.

Below is the result of a 'ls -l' of my /boot directory.

total 20496
-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 36780 Mar 27 2018 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 Feb 20 2018 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
drwxr-xr-x 2 root root 8192 Feb 20 2018 System Volume Information

can you post output of df ?

Here you go

/dev/root       61197932 4971144  56210404   9% /
devtmpfs          469532       0    469532   0% /dev
tmpfs             473864     632    473232   1% /dev/shm
tmpfs             473864   24692    449172   6% /run
tmpfs               5120       4      5116   1% /run/lock
tmpfs             473864       0    473864   0% /sys/fs/cgroup
/dev/mmcblk0p1     64456   21344     43112  34% /boot
tmpfs              94776      36     94740   1% /run/user/1000

thanks, sudo apt update; sudo apt upgrade does not upgrade your system?

I ran apt update & upgrade right after I had the error and didn't receive an error that I remember. However, I don't remember it upgrading anything that looked like it was part of the system.