Issue description

When downloading the aarch64 sdcard image, the /etc/nixos/configuration.nix provided there imports nixos/modules/installer/cd-dvd/sd-image-aarch64.nix, which imports nixos/modules/profiles/installation-device.nix, which (among others) enables the nixos manual on tty8, force-enables the documentation, disabes sudo and udisks, disables sshd on bootup and enables automatic login as root without password.

We probably want to provide a different configuration.nix in /etc/nixos/configuration.nix not importing installer-related things (or at least make it easy to disable them).

Steps to reproduce

• Download the aarch64 sdcard image
• flash it to a raspberry pi3
• edit /etc/nixos/configuration.nix (add services.openssh.enable = true for example)
• call nixos-rebuild switch
• wonder why it's not started

Technical details

nixos-sd-image-19.09pre183392.83ba5afcc96-aarch64-linux.img

cc @NinjaTrappeur