Fixing error on new CLI/LIB: cp: no such file or directory

On latest versions of CLI and LIB a `cp: no such file or directory` error was being displayed on the console after adding firefoxos platform.

This was happening because cordova's `platform add` code tries to copy `cordova.js` file from `www` to `platform_www`([code](https://github.com/apache/cordova-lib/commit/3695d6fef8998929189a36681bbe1f99c13f7887)), see [this change](https://github.com/apache/cordova-lib/commit/3695d6fef8998929189a36681bbe1f99c13f7887) for more info. In https://github.com/apache/cordova-firefoxos/commit/72c62a09060a9485d77fc93d84118f1e78706b5a we started moving `cordova.js` into `platform_www` as `prepare` would copy it to `www`.

Fix is to copy `cordova.js` to `www` and let it be copied to `platform_www` by `platform add`.