This enforces presence of the trezor-common submodule, copies coins.json to the
package directory (from where we can install it with bdist) and if possible,
regenerates protobuf messages.
That currently doesn't work on Windows, because it's a shell script. Also it
relies on presence of `protoc` protobuf compiler. Therefore the regeneration
step is optional and converted protobuf messages should still be commited to
this repo.
coins.json, OTOH, is gitignored in trezorlib, and must be copied from
trezor-common every time. This works because sdist includes the vendor
directory.
