This isn't an initramfs-tools specific issue, moving to Ubuntu project. The short answer is you need to resize your partitions, to do that you will need to boot a live CD or similar and use the partitioner to shrink the main partition and expand /boot.
** Package changed: initramfs-tools (Ubuntu) => ubuntu -- You received this bug notification because you are a member of Ubuntu Touch seeded packages, which is subscribed to initramfs-tools in Ubuntu. https://bugs.launchpad.net/bugs/1472757 Title: How do I expand the free space in "/boot" ? Status in Ubuntu: New Bug description: can't expand BOOT dir to install updates or upgrades. To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+bug/1472757/+subscriptions -- Mailing list: https://launchpad.net/~touch-packages Post to : touch-packages@lists.launchpad.net Unsubscribe : https://launchpad.net/~touch-packages More help : https://help.launchpad.net/ListHelp