doc: Add note about using a pre-build rootfs
@@ -41,6 +41,37 @@ and mainly, installation steps.
Fear not, look for your particular device on the <<devices/index.adoc#,devices list>>
page, will likely contain the necessary instructions.
+=== Using a pre-built `system.img`
+
+When bootstrapping yourself up, without an aarch64-linux system, it may be
+easier to deal with some systems if you use a pre-built `rootfs` rather than
+rely on the dummy basic `rootfs`. For those systems, you can download a
+link:https://hydra.nixos.org/job/mobile-nixos/unstable/examples-demo.aarch64-linux.rootfs[pre-built rootfs],
+and configure the system build to use it. Assuming it is next to the
+`local.nix` file in your Mobile NixOS checkout.
+
+```nix
+# local.nix
+{ pkgs, ... }:
+
+let
+ fromPath = input: pkgs.runCommandNoCC "system.img" {
+ filename = "system.img";
+ filesystemType = "ext4";
+ } ''
+ mkdir -p "$out"
+ cp ${input} "$out/$filename"
+ '';
+in
+{
+ system.build.rootfs = fromPath ./system.img;
+}
+```
+
+Note that for Android-based devices, you can simply use the `system.img`. There
+is no need to configure it as such. This is most useful for _depthcharge_ and
+_u-boot_ systems.
+
== Customizing
