feat(configurations)!: flatten down, introduce meta.nix

Flatten down directory structure:
- From: `./hosts/${configuration-type}/${system}/{configuration,deploy}.nix`
-   To: `./hosts/${configuration-type}/{meta,configuration}.nix`
Keep `system` and `deploy-rs` config in `meta.nix`
Update `flake.lock`

hosts/nix-on-droid/cheetah/meta.nix

@@ -0,0 +1,3 @@
+{
+  system = "aarch64-linux";
+}