Add nix flake support

Now that nix 2.4 is out, flakes are no longer experimental.

flake.lock

@@ -0,0 +1,43 @@
+{
+  "nodes": {
+    "flake-utils": {
+      "locked": {
+        "lastModified": 1638122382,
+        "narHash": "sha256-sQzZzAbvKEqN9s0bzWuYmRaA03v40gaJ4+iL1LXjaeI=",
+        "owner": "numtide",
+        "repo": "flake-utils",
+        "rev": "74f7e4319258e287b0f9cb95426c9853b282730b",
+        "type": "github"
+      },
+      "original": {
+        "owner": "numtide",
+        "repo": "flake-utils",
+        "type": "github"
+      }
+    },
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1638910453,
+        "narHash": "sha256-fofA4tsAAdNgE+9Py0VsVbyX2ZQkgV+CVqQKGBA/dLE=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "f225322e3bea8638304adfcf415cd11de99f2208",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "nixpkgs-unstable",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "root": {
+      "inputs": {
+        "flake-utils": "flake-utils",
+        "nixpkgs": "nixpkgs"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}

flake.nix

@@ -0,0 +1,11 @@
+{
+  description = "# Zephyr™ Mechanical Keyboard (ZMK) Firmware";
+
+  inputs.nixpkgs.url = "github:NixOS/nixpkgs/nixpkgs-unstable";
+  inputs.flake-utils.url = "github:numtide/flake-utils";
+
+  outputs = { self, nixpkgs, flake-utils }:
+    flake-utils.lib.eachDefaultSystem (system:
+      let pkgs = nixpkgs.legacyPackages.${system};
+      in { devShell = import ./shell.nix { inherit pkgs; }; });
+}

shell.nix

@@ -6,14 +6,14 @@ let inherit (lib) ;
 in mkShell {
   buildInputs = [
     cmake
-    ninja
-    python3
     ccache
+    ninja
     dtc
     dfu-util
     gcc-arm-embedded
+    python3
     python38Packages.west
-    python38Packages.pyelftools
+    python38Packages.pip
   ];
 
   shellHook = ''