Add nix stuff

.envrc

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+use nix

.gitignore

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+.direnv/*

shell.nix

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+{ pkgs ? import <nixpkgs> {} }:
+  pkgs.mkShell {
+    nativeBuildInputs = with pkgs.buildPackages; [
+      python3.pkgs.pyserial
+      (writeShellScriptBin "flash-thb1" ''
+        python3 rdwr_phy62x2.py -p /dev/ttyUSB0 -b 115200 -e -r wh ./bin/BOOT_THB1_v*.hex
+      '')
+    ];
+}