Add common code for make.py to the shared resources

[pietroalbini]

The FLS's make.py script is needed in other repositories as well, and it wouldn't make sense to duplicate it. This PR creates a make_common.py script that contains most of the logic of make.py, so that make.py can be reduced to:

#!/usr/bin/env python3
# SPDX-License-Identifier: MIT OR Apache-2.0
# SPDX-FileCopyrightText: Ferrous Systems and AdaCore

import os
import subprocess
import sys

root = os.path.abspath(os.path.dirname(__file__))
subprocess.run(
    ["git", "submodule", "update", "--init"],
    check=True,
    cwd=root,
)

sys.path.insert(0, "shared")
import make_common
make_common.main(root)