Raspberry Pi 1-3 image building toolchain (amongst other repos) on the project page:

https://github.com/nerves-project

https://github.com/nerves-project/nerves_system_rpi

https://github.com/nerves-project/nerves_system_rpi2

https://github.com/nerves-project/nerves_system_rpi3

Check out examples here:

https://github.com/nerves-project/nerves-examples