Comment 4 for bug 1765628

flash-kernel is the thing that puts the bootloader and bootloader configuration in place at the end of installations (the equivalent to grub-install on x86).
It is the interface/API you need if you want to use ubiquity or debian-intaller.
It is also hooked up directly with the kernel debs and automatically run after a kernel update (to adjust bootloader variables, write things to flash on systems where thats needed etc).

while you can indeed go completely without debian-installer/ubiquity and use self-expanding pre-installed images, you would need to special-case a bunch of expected distro-defaults in the images... and would have to maintain this patchset.

my personal preference would also be to chainload grub from u-boot like opensuse and fedora do to implement UEFI, but this requires a lot of integration work in various places of the distro. this should be done as a general solution for all armhf installs across the board and needs a bit more planning than just throwing together a rpi image ... that said, as a proof of concept, a pi image would indeed be a good start.

note though that in the end you will still need some generic api/interface for the installers if you want to use them. which will make you end up with a flash-kernel-like tool again.