Introduce Release Sign Tool.

[minosgalanakis]

This pr introduces the release sign tool, as discussed in the design review meeting.

It is a standalone tool serving a web-app and using gpg tool to sign and validate release tarballs.

How to test locally:

cd tools/release_sign_tool
python -m vevn rel_sign_env && rel_sign_env/bin/activate && pip install -r release_sign_requirements.txt
MBEDTLS_FW_SIGN_SERVER_IP=X.X.X.X && python release_sign_server.py

This PR supersedes PR147