I didn't know Nix had support for embedded systems. Where are the images defined?

What do you mean by image? In Nix you instantiate a NixOS system and specify the architecture. There are a number of tools to build a disk image from a NixOS system.

$ my-arm-system = nixpkgs.lib.nixosSystem { system = "aarch64-linux"; modules = [ ./configuration.nix ]; };

$ :b arm-system.config.system.build.images.iso