I used mcuboot to create a bootloader in riot. It’s very young. I just today managed to get it to boot an image. It would have a long time ago but I was trying to boot with misaligned vector offsets (my nrf requires a certain alignment and I was just putting it wherever).
Totally out of my comfort zone, so I might be wrong about this, but don’t we already support mcuboot? Maybe some of the people behind it (@Kaspar, @kYc0o, @aabadie, …) can shed some light on this.
Normally it should be working but only on nrf52 boards as it needs some definitions e.g. see here. But AFAIK the application does not work, I tried all the way back to https://github.com/RIOT-OS/RIOT/pull/11083 and it did not work (was reported that way at the time).