This PR attempts to implement the _thread module on the Zephyr port. Three important remarks about this PR:

1. Due to the fact that we are still using a rather old version of Zephyr, CONFIG_DYNAMIC_THREAD is not available and therefore the stack for threads cannot be allocated dynamically, only at compile time. So for the time being and for the purpose of these PR, a maximum of 4 Zephyr threads (besides the main thread) can be created. Once we manage to update to the latest version of Zephyr this won't be a problem anymore.

2. For some reason, threads get blocked forever before calling mp_thread_start(); inside thread_entry() (see modthread.c, line 174), because MP_THREAD_GIL_ENTER() never returns. I'm unable to understand why this is happening, since this implementation of mpthreadport for the Zephyr port is following the same approach used by the ESP32 port. I suspect there's some kind of incompatibility with the rest of the Micropython options enabled for the Zephyr port, and I hope that @dpgeorge can shed some light in here. For this reason MICROPY_PY_THREAD_GIL is disabled at the moment. This obviously needs to be fixed before this PR is merged.

3. Some of the tests in the tests/thread folder run successfully, and some other don't most likely because MICROPY_PY_THREAD_GIL is not enabled.

A simple test that runs OK:

import _thread
import time

def thread_entry(num):
    for i in range(3):
        time.sleep(1 + (num * 0.5))
        print("Thread {} running...".format(num))

_thread.start_new_thread(thread_entry, (0,))
_thread.start_new_thread(thread_entry, (1,))
_thread.start_new_thread(thread_entry, (2,))
_thread.start_new_thread(thread_entry, (3,))