Move scripts/code_style.py to the framework

[mpg]

This script has slightly diverged between development and 3.6. This task is to:

• reconcile the two branches;
• move the script to the framework;
• adapt the path everywhere the script is invoked.

Note: this task is only about mbedtls and the framework. There is a separate task for adapting this script to tf-psa-crypto: Mbed-TLS/TF-PSA-Crypto#48 so that is out of scope here.