"ERROR Installing the upgrades failed!" because of too small boot partition

I am using Freedombox-testing with "automatic upgrades" enabled. I am still stuck with plinth version 0.36.0 because of upgrade errors.

I have attached my /plinth/sys/upgrades/upgrade/ log as upgrade-log-with-errors.txt.

The last entry of /var/log/unattended-upgrades/unattended-upgrades-dpkg.log is

Log started: 2018-09-12  06:51:43
quota not working (qgroup not set)
(Reading database ... ^M(Reading database ... 5%^M(Reading database ... 10%^M(Reading database ... 15%^M(Reading database ... 20%^M(Reading database ... 25%^M^M(Reading database ... 30%^M(Reading database ... 35%^M(Reading database ... 40%^M(Reading database ... 45%^M(Reading database ... 50%^M(Reading database ... 55%^M(Reading database ... 60%^M(Reading database ... 65%^M(Reading database ... 70%^M(Reading database ... 75%^M(Reading database ... 80%^M(Reading database ... 85%^M(Reading database ... 90%^M(Reading database ... 95%^M(Reading database ... 100%^M(Reading database ... 75469 files and directories currently installed.)
Preparing to unpack .../iso-codes_4.1-1_all.deb ...
Unpacking iso-codes (4.1-1) over (3.79-1) ...
Setting up iso-codes (4.1-1) ...
Setting up initramfs-tools (0.132) ...
update-initramfs: deferring update (trigger activated)
Processing triggers for initramfs-tools (0.132) ...
update-initramfs: Generating /boot/initrd.img-4.17.0-3-armmp-lpae

gzip: stdout: No space left on device
E: mkinitramfs failure cpio 141 gzip 1
update-initramfs: failed for /boot/initrd.img-4.17.0-3-armmp-lpae with 1.
dpkg: error processing package initramfs-tools (--configure):
 installed initramfs-tools package post-installation script subprocess returned error exit status 1
Errors were encountered while processing:
 initramfs-tools
quota not working (qgroup not set)
Log ended: 2018-09-12  06:54:41

And in fact, /plinth/sys/storage/ shows:

I got this partitioning from freedombox-testing-free_latest_a20-olinuxino-lime2-armhf.img.xz. What should I do now? I guess I could resize the partitions with GParted.

This certainly is the same problem that was first reported to the mailing list on 2018-07-30 by Dietmar as "WARNING package upgradable but fails to be marked for upgrade ()", then confirmed by Daddy and me.