stmhal: Initial implementation of multithreading, currently disabled.
This patch brings the _thread module to stmhal/pyboard. There is a very simple round-robin thread scheduler, which is disabled if there is only one thread (for efficiency when threading is not used). The scheduler currently switches threads at a rate of 250Hz using the systick timer and the pend-SV interrupt. The GIL is disabled so one must be careful to use lock objects to prevent concurrent access of objects. The threading is disabled by default, one can enabled it with the config option MICROPY_PY_THREAD to test it out.