Rework the Makefiles to be compatible with binary packages.

generator/protoc-gen-nanopb

@@ -0,0 +1,12 @@
+#!/bin/sh
+
+# This file is used to invoke nanopb_generator.py as a plugin
+# to protoc on Linux and other *nix-style systems.
+# Use it like this:
+# protoc --plugin=nanopb=..../protoc-gen-nanopb --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.
+
+exec python $(dirname $0)/nanopb_generator.py --protoc-plugin