Now we don't need build_protobuf anymore and this is usable
by both core and python-trezor
(as well as generating custom protobufs from other sources)
We still need protoc, unfortunately, but pb2py now calls it
by itself. (little more robustly; instead of generated
python classes, it uses the FileDescriptorSet output which
is parsable by a built-in protobuf class)
To support the script, messages.proto and types.proto must
set a common package. Also there is currently no support for
compiling more than one proto file, we depend on the fact
that messages.proto import types.proto.
(if this is needed, it should be relatively simple to add,
simply pass more than one file to the embedded protoc call)
