tools/nix: Add Nix flake for reproducible dev environment

This commit introduces a Nix flake (flake.nix and flake.lock) to provide
a reproducible development shell for NuttX using Nix and flakes.

- Adds a devShell with all required build tools and dependencies for NuttX,
  including CMake, Ninja, GNU Make, clang-tools, ARM toolchain, and other
  utilities (automake, bison, flex, genromfs, gettext, gperf, kconfig-frontends,
  libelf, expat, gmp, isl, libmpc, mpfr, ncurses, zlib, kconfiglib).
- Uses flake-utils to support multiple platforms.
- Sets up a shell hook to enable CMake compile commands and provide a welcome
  message.
- Based on nixpkgs `nixos-unstable` for up-to-date packages.

Purpose:
- Simplifies onboarding and ensures a consistent build environment across
  contributors.
- Reduces "works on my machine" issues by pinning dependencies.

Tested by running `nix develop` and building NuttX on x86_64-linux.

No impact on runtime or target builds; this only affects developer tooling.

For more details, see the new flake.nix file.

Signed-off-by: Côme VINCENT <44554692+comejv@users.noreply.github.com>

flake.lock

@@ -0,0 +1,61 @@
+{
+  "nodes": {
+    "flake-utils": {
+      "inputs": {
+        "systems": "systems"
+      },
+      "locked": {
+        "lastModified": 1731533236,
+        "narHash": "sha256-l0KFg5HjrsfsO/JpG+r7fRrqm12kzFHyUHqHCVpMMbI=",
+        "owner": "numtide",
+        "repo": "flake-utils",
+        "rev": "11707dc2f618dd54ca8739b309ec4fc024de578b",
+        "type": "github"
+      },
+      "original": {
+        "owner": "numtide",
+        "repo": "flake-utils",
+        "type": "github"
+      }
+    },
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1753250450,
+        "narHash": "sha256-i+CQV2rPmP8wHxj0aq4siYyohHwVlsh40kV89f3nw1s=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "fc02ee70efb805d3b2865908a13ddd4474557ecf",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "nixos-unstable",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "root": {
+      "inputs": {
+        "flake-utils": "flake-utils",
+        "nixpkgs": "nixpkgs"
+      }
+    },
+    "systems": {
+      "locked": {
+        "lastModified": 1681028828,
+        "narHash": "sha256-Vy1rq5AaRuLzOxct8nz4T6wlgyUR7zLU309k9mBC768=",
+        "owner": "nix-systems",
+        "repo": "default",
+        "rev": "da67096a3b9bf56a91d16901293e51ba5b49a27e",
+        "type": "github"
+      },
+      "original": {
+        "owner": "nix-systems",
+        "repo": "default",
+        "type": "github"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}

flake.nix

@@ -0,0 +1,70 @@
+{
+  description = "NuttX : devShell flake";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+    flake-utils.url = "github:numtide/flake-utils";
+  };
+
+  outputs =
+    {
+      self,
+      nixpkgs,
+      flake-utils,
+      ...
+    }:
+    flake-utils.lib.eachDefaultSystem (
+      system:
+      let
+        pkgs = import nixpkgs { inherit system; };
+      in
+      {
+        devShells.default = pkgs.mkShell {
+          buildInputs = [
+            # Build tools
+            pkgs.cmake
+            pkgs.ninja
+            pkgs.gnumake
+            pkgs.clang-tools
+
+            # ARM toolchain
+            pkgs.gcc-arm-embedded
+
+            # NuttX dependencies
+            pkgs.automake
+            pkgs.bison
+            pkgs.flex
+            pkgs.genromfs
+            pkgs.gettext
+            pkgs.gperf
+            pkgs.kconfig-frontends
+            pkgs.libelf
+            pkgs.expat.dev
+            pkgs.gmp.dev
+            pkgs.isl
+            pkgs.libmpc
+            pkgs.mpfr.dev
+            pkgs.ncurses.dev
+            pkgs.zlib
+            pkgs.python313Packages.kconfiglib
+
+            # NuttX Documentation
+            pkgs.sphinx
+            pkgs.python313Packages.sphinx_rtd_theme
+            pkgs.python313Packages.myst-parser
+            pkgs.python313Packages.sphinx-tabs
+            pkgs.python313Packages.sphinx-autobuild
+            pkgs.python313Packages.sphinx-copybutton
+            pkgs.python313Packages.sphinx-togglebutton
+            pkgs.python313Packages.pytz
+            pkgs.python313Packages.importlib-metadata
+            pkgs.python313Packages.sphinx-design
+          ];
+          shellHook = ''
+            export CMAKE_EXPORT_COMPILE_COMMANDS=ON
+            echo "Welcome to NuttX devShell"
+          '';
+        };
+      }
+    );
+}