Unify root directory identification in python scripts

[gilles-peskine-arm]

We've accumulated at least three different methods to determine the mbedtls root directory in Python scripts: check_repo_path, chdir_to_root and guess_mbedtls_root. The goal of this task is to unify them, so that all scripts behave in a uniform manner when not run from the mbedtls root.

Note that for any solution involving chdir, we need to take care about script arguments that are file or directory relative paths: they should be interpreted relative to the current directory when the script is invoked, not relative to the mbedtls root.