move the codeName to /.codeName

Make the codeName globally accessible in the repo. The release is not
only for NixOS anymore.

.codeName

@@ -0,0 +1 @@
+Koi

lib/trivial.nix

@@ -98,6 +98,9 @@ rec {
   # The current nixpkgs version number as string.
   release = lib.strings.fileContents ../.version;
 
+  # Note: the first letter is bumped on every release.  It's an animal.
+  codeName = lib.strings.fileContents ../.codeName;
+
   # The current nixpkgs version suffix as string.
   versionSuffix =
     let suffixFile = ../.version-suffix;

nixos/modules/misc/version.nix

@@ -43,6 +43,7 @@ in
     nixos.codeName = mkOption {
       readOnly = true;
       type = types.str;
+      default = lib.trivial.codeName;
       description = "The NixOS release code name (e.g. <literal>Emu</literal>).";
     };
 
@@ -79,9 +80,6 @@ in
       version = mkDefault (cfg.release + cfg.versionSuffix);
       revision      = mkIf (pathIsDirectory gitRepo) (mkDefault            gitCommitId);
       versionSuffix = mkIf (pathIsDirectory gitRepo) (mkDefault (".git." + gitCommitId));
-
-      # Note: the first letter is bumped on every release.  It's an animal.
-      codeName = "Koi";
     };
 
     # Generate /etc/os-release.  See