add packages to Nix flake config, move to repo root

This way we can actually build and run a node using just:
```sh
nix run github:status-im/nimbus-eth2?submodules=1
```
The `?submodules=1` part should eventually not be necessary.
For more details see:
NixOS/nix#4423

Signed-off-by: Jakub Sokołowski <jakub@status.im>

.gitignore

@@ -65,6 +65,7 @@ geth-*.zip
 # generated during Nim compilation
 *.nim.generated.nim
 
+/result
 /dist
 /benchmark_results
 /.update.timestamp

flake.nix

@@ -0,0 +1,35 @@
+{
+  description = "nimbus-eth2";
+
+  inputs.nixpkgs.url = github:NixOS/nixpkgs/master;
+
+  outputs = { self, nixpkgs }:
+    let
+      stableSystems = [
+        "x86_64-linux" "aarch64-linux" "armv7a-linux"
+        "x86_64-darwin" "aarch64-darwin"
+        "x86_64-windows"
+      ];
+      forEach = nixpkgs.lib.genAttrs;
+      forAllSystems = forEach stableSystems;
+      pkgsFor = forEach stableSystems (
+        system: import nixpkgs { inherit system; }
+      );
+    in rec {
+      packages = forAllSystems (system: let
+        build = pkgsFor.${system}.callPackage ./nix/default.nix {
+          inherit stableSystems; src = self;
+        };
+      in rec {
+        beacon_node      = build.override { targets = ["nimbus_beacon_node"]; };
+        signing_node     = build.override { targets = ["nimbus_signing_node"]; };
+        validator_client = build.override { targets = ["nimbus_validator_client"]; };
+
+        default = beacon_node;
+      });
+
+      devShells = forAllSystems (system: {
+        default = pkgsFor.${system}.callPackage ./nix/shell.nix { };
+      });
+    };
+}

nix/README.md

@@ -0,0 +1,29 @@
+# Usage
+
+## Shell
+
+A development shell can be started using:
+```sh
+nix develop
+```
+
+## Building
+
+To build a beacon node you can use:
+```sh
+nix build .?submodules=1
+```
+The `?submodules=1` part should eventually not be necessary.
+For more details see:
+https://github.com/NixOS/nix/issues/4423
+
+It can be also done without even cloning the repo:
+```sh
+nix build github:status-im/nimbus-eth2?submodules=1
+```
+
+## Running
+
+```sh
+nix run github:status-im/nimbus-eth2?submodules=1
+```

nix/default.nix

@@ -0,0 +1,70 @@
+{
+  stdenv, darwin, lib, nim1, cmake, which, writeScriptBin,
+  src ? ../.,
+  # Options: nimbus_light_client, nimbus_validator_client, nimbus_signing_node, all
+  targets ? ["nimbus_beacon_node"],
+  # These are the only platforms tested in CI and considered stable.
+  stableSystems ? [
+    "x86_64-linux" "aarch64-linux" "armv7a-linux"
+    "x86_64-darwin" "aarch64-darwin"
+    "x86_64-windows"
+  ],
+}:
+
+# Nim version(s) that are known to be stable
+# See: https://github.com/status-im/nimbus-build-system/commits/master/vendor
+assert (
+  lib.assertMsg
+  (builtins.elem nim1.version ["1.6.16" "1.6.18"])
+  "Unsupported Nim version: ${nim1.version}"
+);
+
+stdenv.mkDerivation rec {
+  pname = "nimbus-eth2";
+  version = "unknown";
+
+  inherit src;
+
+  # Fix for Nim compiler calling 'git rev-parse' and 'lsb_release'.
+  nativeBuildInputs = let
+    fakeGit = writeScriptBin "git" "echo ${version}";
+    fakeLsbRelease = writeScriptBin "lsb_release" "echo nix";
+  in
+    [ fakeGit fakeLsbRelease nim1 which cmake ]
+    ++ lib.optionals stdenv.isDarwin [ darwin.cctools ];
+
+  enableParallelBuilding = true;
+
+  # Disable CPU optmizations that make binary not portable.
+  NIMFLAGS = "-d:disableMarchNative -d:git_revision_override=${version}";
+
+  makeFlags = targets ++ ["USE_SYSTEM_NIM=1"];
+
+  # Generate the nimbus-build-system.paths file.
+  configurePhase = ''
+    patchShebangs scripts vendor/nimbus-build-system/scripts > /dev/null
+    make nimbus-build-system-paths
+  '';
+
+  installPhase = ''
+    mkdir -p $out/bin
+    rm -f build/generate_makefile
+    cp build/* $out/bin
+  '';
+
+  meta = with lib; {
+    homepage = "https://nimbus.guide/";
+    downloadPage = "https://github.com/status-im/nimbus-eth2/releases";
+    changelog = "https://github.com/status-im/nimbus-eth2/blob/stable/CHANGELOG.md";
+    description = "Nimbus is a lightweight client for the Ethereum consensus layer";
+    longDescription = ''
+      Nimbus is an extremely efficient consensus layer client implementation.
+      While it's optimised for embedded systems and resource-restricted devices --
+      including Raspberry Pis, its low resource usage also makes it an excellent choice
+      for any server or desktop (where it simply takes up fewer resources).
+    '';
+    license = with licenses; [asl20 mit];
+    mainProgram = "nimbus_beacon_node";
+    platforms = stableSystems;
+  };
+}

installer/nix/shell.nix → nix/shell.nix

@@ -6,6 +6,7 @@
 # at your option. This file may not be copied, modified, or distributed except according to those terms.
 
 { pkgs ? import <nixpkgs> {}}:
+
 let
   mkdocs-packages = ps: with ps; [
     mkdocs
@@ -14,11 +15,9 @@ let
     pymdown-extensions
   ];
   mkdocs-python = pkgs.python3.withPackages mkdocs-packages;
-in
-with pkgs;
-mkShell {
+in pkgs.mkShell {
 
-  buildInputs = [
+  buildInputs = with pkgs; [
     figlet
     git
     git-lfs
@@ -43,6 +42,7 @@ mkShell {
     # For the purposes of compiling Nimbus, this behavior is not desired:
     export NIX_ENFORCE_NO_NATIVE=0
     export USE_SYSTEM_GETOPT=1
+    export MAKEFLAGS="-j$NIX_BUILD_CORES"
 
     figlet "Welcome to Nimbus-eth2"
   '';