flutter-engine: allow local version pinning
Created by: kwavnet
currently the local patches in flutter-engine do only fit for the proposed default version of FLUTTER_SDK_TAG. If one would want to locally pin the version to something different one or more patches wouldn’t apply. Fix this by weak defining a new variable called FLUTTER_ENGINE_PATCHES which can be easily overridden in a local setup or a layer on top of meta-flutter to have the right set of patches stored there