feat!(flake): convert to fs-defined configurations

Place all configurations in `machines/${manager}/${system}/${hostname}/...`:
    - `${manager}` - One of `nixos`, `nix-on-droid`, `nix-darwin` or `home-manager`
    - `${system}` - A system's architecture (can be many)
    - `${hostname}` - A system's hostname (can be many)

The flake now automatically generates the needed configurations based on the above structure.

It only generates configurations for valid directory structures:
    - It wouldn't generate a NixOS config if there isn't a `configuration.nix` file
    - ...

machines/nix-on-droid/aarch64-linux/cheetah/configuration.nix

@@ -0,0 +1,42 @@
+{ inputs, outputs, lib, config, pkgs, ... }:
+
+{
+  environment.packages = with pkgs; [];
+
+  # Backup etc files instead of failing to activate generation if a file already exists in /etc
+  environment.etcBackupExtension = ".bak";
+
+  # Remove motd
+  environment.motd = null;
+
+  # # Set $EDITOR
+  # environment.variables.EDITOR = "nvim";
+
+  user.shell = "${pkgs.zsh}/bin/zsh";
+
+  # Read the changelog before changing this value
+  system.stateVersion = "22.05";
+
+  nix.extraOptions = ''
+    experimental-features = nix-command flakes
+  '';
+
+  time.timeZone = "Europe/Sofia";
+
+  terminal.font =
+    let
+      firacode = pkgs.nerdfonts.override {
+        fonts = [ "FiraCode" ];
+      };
+      fontPath = "share/fonts/truetype/NerdFonts/Fira Code Regular Nerd Font Complete Mono.ttf";
+    in
+      "${firacode}/${fontPath}";
+
+  home-manager = {
+    config = ./home.nix;
+    backupFileExtension = "hm-bak";
+    # useGlobalPkgs = true;
+
+    extraSpecialArgs = { inherit inputs; };
+  };
+}