Comment 11 for bug 1337538

Basically, currently the Debian/Ubuntu approach is that different platforms need different boot setups; e.g. legacy BIOS vs. EFI, ARM vs x86, this or that specific platform needs this or that specific boot package.

The way the right packages for boot get pulled on live images and on debian-installer images is by running the logic in udeb packages (flash-kernel-installer.postinst).

Either you dont need a boot update solution, and then you should not installed it as outlined in:
https://bugs.launchpad.net/ubuntu/+source/flash-kernel/+bug/1160488/comments/3
or you do need a boot files update solution, and then if you opted to pull flash-kernel you should call its logic to pick which extra packages are needed for your platform.

This aint very convenient, but it's the current design; it's shared with Debian and it's an area where we're trying to avoid divergence (installer stuff).