Messages rebuild

Install protoc for your distribution. Requirements:

protobuf-compiler
libprotobuf-dev
python

Soft requirement: Python 3, can be easily installed with pyenv. If Python 3 is used there are no additional python dependencies.

Since Cmake 3.12 the FindPython module is used to locate the Python interpreter in your system. It preferably searches for Python 3, if none is found, it searches for Python 2.

Lower version of the cmake uses another module which does not guarantee ordering. If you want to override the selected python you can do it in the following way:

``bash export TREZOR_PYTHON=which python3`

### Python 2.7+
Python 3 has `tempfile.TemporaryDirectory` available but Python 2 lacks
this class so the message generation code uses `backports.tempfile` package
bundled in the repository.
The minimal Python versions are 2.7 and 3.4
### Regenerate messages
```bash
cd src/device_trezor/trezor
python tools/build_protob.py

The messages regeneration is done also automatically via cmake.