nixos/activation: add fisrt-start activation script support

[danbst]

The problem: I had a RPi 3, and had no keyboard, no ethernet plug and no monitor. The only two ways to talk to it were via sdcard and WiFi.

Ideally it should be possible to:

• write nixos image to sdcard
• add /etc/nixos/configuration.nix to sdcard root's filesystem
• add startup script somewhere (which should setup WiFi, start sshd and run nixos-rebuild boot)
• let nixos run this script on first boot

It's impossible to build a custom image for RPi without having aarch64 machine, so there should be a standard way to customize sdcard system. I think something similar is already done for Amazon images.

[symphorien]

Maybe you can put your startup script in a systemd unit: from man systemd.unit:

           ConditionFirstBoot= takes a boolean argument. This condition may be used to
           conditionalize units on whether the system is booting up with an unpopulated /etc
           directory (specifically: an /etc with no /etc/machine-id). This may be used to
           populate /etc on the first boot after factory reset, or when a new system
           instance boots up for the first time.

[stale]

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[dfrankland]

I followed the systemd approach and was able to get this to work:
https://github.com/dfrankland/nixos-rpi-sd-image/blob/50bee6fe136daf5450323f154695348cb49de831/nixos-rpi-sd-image/sd-image-init.nix

[stale]

I marked this as stale due to inactivity. → More info