minimal NixOS boot to login prompt

note that we pin mobile-nixos and nixpkgs: the nixpkgs pin might be
unnecessary, however the mobile-nixos pin is essential because
the uboot target creates a non-bootable image shortly after the chosen
commit. mobile-nixos is in the process of switching away from uboot and to tow-boot.

README.md

@@ -0,0 +1,14 @@
+build from any nix machine with:
+```sh
+nix build './#pinephone-img'
+```
+
+if nix complains about some "experimental features", then add to the host's nix config: `nix.extraOptions = "experimental-features = nix-command flakes";`
+
+flash with:
+```sh
+sudo dd if=$(readlink result) of=/dev/sdb bs=4M oflag=direct conv=sync status=progress
+```
+
+then insert the SD card, battery into your pinephone and hold the power button for a few seconds until the power LED turns red.
+after releasing the power button, the LED should turn yellow, then green. you'll see the "mobile NixOS" splash screen and then be dropped into a TTY login prompt.

flake.lock

@@ -0,0 +1,44 @@
+{
+  "nodes": {
+    "mobile-nixos": {
+      "flake": false,
+      "locked": {
+        "lastModified": 1653513225,
+        "narHash": "sha256-DjIrBfb3cbLG15cWKpSIfBxorXCMWVBeFOc1K7HOzw4=",
+        "owner": "nixos",
+        "repo": "mobile-nixos",
+        "rev": "efbe2c3c5409c868309ae0770852638e623690b5",
+        "type": "github"
+      },
+      "original": {
+        "owner": "nixos",
+        "repo": "mobile-nixos",
+        "rev": "efbe2c3c5409c868309ae0770852638e623690b5",
+        "type": "github"
+      }
+    },
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1653060744,
+        "narHash": "sha256-kfRusllRumpt33J1hPV+CeCCylCXEU7e0gn2/cIM7cY=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "dfd82985c273aac6eced03625f454b334daae2e8",
+        "type": "github"
+      },
+      "original": {
+        "id": "nixpkgs",
+        "rev": "dfd82985c273aac6eced03625f454b334daae2e8",
+        "type": "indirect"
+      }
+    },
+    "root": {
+      "inputs": {
+        "mobile-nixos": "mobile-nixos",
+        "nixpkgs": "nixpkgs"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}

flake.nix

@@ -0,0 +1,25 @@
+{
+  inputs = {
+    # nixpkgs.url = "nixpkgs/nixos-22.05";
+    nixpkgs.url = "nixpkgs/dfd82985c273aac6eced03625f454b334daae2e8";
+    mobile-nixos = {
+      # url = "github:nixos/mobile-nixos";
+      url = "github:nixos/mobile-nixos/efbe2c3c5409c868309ae0770852638e623690b5";
+      flake = false;
+    };
+  };
+
+  outputs = { self, nixpkgs, mobile-nixos }: {
+    pinephone-img = (nixpkgs.lib.nixosSystem {
+      system = "aarch64-linux";
+      modules = [
+        (import "${mobile-nixos}/lib/configuration.nix" {
+          device = "pine64-pinephone";
+        })
+        ({ ... }: {
+          nixpkgs.config.allowUnfree = true;
+        })
+      ];
+    }).config.mobile.outputs.u-boot.disk-image;
+  };
+}