flashtool: mark as broken

The sources are no longer obtainable from the URL specified in the
expression and the package has no maintainers.

pkgs/development/mobile/flashtool/default.nix

···
     license = stdenv.lib.licenses.unfreeRedistributableFirmware;
     platforms = stdenv.lib.platforms.linux;
     hydraPlatforms = stdenv.lib.platforms.none;
+    broken = true;
   };
 }