retroarch: add support for declarative settings

Add a new optional 'settings' attrset to the wrapper derivation, which
gets serialized to a file and passed to RetroArch as --appendconfig= at
runtime. This allows overriding settings from
~/.config/retroarch/retroarch.cfg (which initially gets created as a
dump of all internal retroarch settings -- stateful and messy).

pkgs/applications/emulators/retroarch/wrapper.nix

···
 , makeWrapper
 , retroarch
 , symlinkJoin
+, runCommand
 , cores ? [ ]
+, settings ? { }
 }:
 
 let
+  settingsPath = runCommand "declarative-retroarch.cfg"
+    {
+      value = lib.concatStringsSep "\n" (lib.mapAttrsToList (n: v: "${n} = \"${v}\"") settings);
+      passAsFile = [ "value" ];
+    }
+    ''
+      cp "$valuePath" "$out"
+    '';
+
   # All cores should be located in the same path after symlinkJoin,
   # but let's be safe here
   coresPath = lib.lists.unique (map (c: c.libretroCore) cores);
-  wrapperArgs = lib.strings.escapeShellArgs
-    (lib.lists.flatten
-      (map (p: [ "--add-flags" "-L ${placeholder "out" + p}" ]) coresPath));
+  wrapperArgs = lib.strings.escapeShellArgs (
+    (lib.lists.flatten (map (p: [ "--add-flags" "-L ${placeholder "out" + p}" ]) coresPath))
+    ++ [ "--add-flags" "--appendconfig=${settingsPath}" ]
+  );
 in
 symlinkJoin {
   name = "retroarch-with-cores-${lib.getVersion retroarch}";

pkgs/top-level/all-packages.nix

···
       (builtins.attrValues libretro);
   };
 
-  wrapRetroArch = { retroarch }:
+  wrapRetroArch = { retroarch, settings ? {} }:
     callPackage ../applications/emulators/retroarch/wrapper.nix
-      { inherit retroarch; };
+      { inherit retroarch settings; };
 
   retroarch = wrapRetroArch {
     retroarch = retroarchBare.override {