nix: init flake.nix

This inits a flake.nix into the repository that exports a convenient Nix shell.
When using `$ nix develop`, users will get a shell with all relevant tools to
build the repository. If one uses a "direnv" integration into their shell, the
environment is automatically loaded.

.gitignore

@@ -47,3 +47,7 @@
 /common-uefi-riscv64
 /decompressor-build
 /stage1.stamp
+
+# Nix
+/.direnv
+/result*