Rework the Makefiles to be compatible with binary packages.

generator/protoc-gen-nanopb.bat

@@ -0,0 +1,11 @@
+:: This file is used to invoke nanopb_generator.py as a plugin
+:: to protoc on Windows.
+:: Use it like this:
+:: protoc --plugin=nanopb=..../protoc-gen-nanopb.bat --nanopb_out=dir foo.proto
+::
+:: Note that if you use the binary package of nanopb, the protoc
+:: path is already set up properly and there is no need to give
+:: --plugin= on the command line.
+
+set mydir=%~dp0
+python %mydir%\nanopb_generator.py --protoc-plugin