fwup: fix darwin build

pkgs/tools/compression/xdelta/default.nix

···
     '';
     homepage = "http://xdelta.org/";
     license = licenses.gpl2Plus;
-    platforms = platforms.linux;
+    platforms = platforms.unix;
   };
 }

pkgs/tools/misc/fwup/default.nix

···
     xdelta
   ];
 
-  doCheck = true;
+  doCheck = !stdenv.isDarwin;
 
   meta = with lib; {
     description = "Configurable embedded Linux firmware update creator and runner";