add build environment files for nixpkgs

.gitmodules

@@ -4,3 +4,6 @@
 [submodule "modules/sdk"]
 	path = modules/sdk
 	url = https://github.com/pfalcon/esp-open-sdk.git
+[submodule "modules/nix"]
+	path = modules/nix
+	url = https://github.com/busti/nixpkgs-esp-dev

README.md

@@ -60,3 +60,11 @@ install `docker` on your platform and buil with
 ```bash
 make firmware_docker -j$(nproc)
 ```
+
+### Build Steps (using nixos)  
+
+In the project root run:
+```
+nix-shell --option sandbox false
+make firmware -j$(nproc)
+```

modules/nix

@@ -0,0 +1 @@
+Subproject commit 403461c810ddab0cf498b24df5b5ff92606e4154

shell.nix

@@ -0,0 +1 @@
+import ./modules/nix/shell-esp-open-rtos.nix {}