examples/jumpdrive: WIP init of a jumpdrive example system

examples/jumpdrive/README.md

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+[JumpDrive](https://github.com/dreemurrs-embedded/Jumpdrive), but with Mobile NixOS
+===================================================================================
+
+What's this?
+------------
+
+This is a system that allows you to present the internal storage of a device
+over the USB connection using Linux USB gadget mode.
+
+
+* * *
+
+With that said, while the project differs enough in implementation, I want
+to acknowledge the obvious inspiration taken from the [JumpDrive](https://github.com/dreemurrs-embedded/Jumpdrive)
+project.

examples/jumpdrive/configuration.nix

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+{ config, lib, pkgs, ... }:
+
+{
+  # There is no mounting here.
+  fileSystems = lib.mkForce {};
+
+  mobile.boot.stage-1.usb = {
+    enable = true;
+    features = [ "mass_storage" ];
+  };
+
+  system.build.rootfs = null;
+
+  mobile.boot.stage-1.networking.enable = true;
+  mobile.boot.stage-1.ssh.enable = true;
+}

examples/jumpdrive/default.nix

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+{ device }:
+
+let
+  system-build = import ../../. {
+    inherit device;
+    configuration = [ (import ./configuration.nix) ];
+  };
+in
+{
+  inherit (system-build) build;
+}