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.
mp_kbd_exception is now considered the standard variable name to hold the
singleton KeyboardInterrupt exception.
This patch also moves the creation of this object from pyb_usb_init() to
main().
This patch consolidates all global variables in py/ core into one place,
in a global structure. Root pointers are all located together to make
GC tracing easier and more efficient.
This allows to implement KeyboardInterrupt on unix, and a much safer
ctrl-C in stmhal port. First ctrl-C is a soft one, with hope that VM
will notice it; second ctrl-C is a hard one that kills anything (for
both unix and stmhal).
One needs to check for a pending exception in the VM only for jump
opcodes. Others can't produce an infinite loop (infinite recursion is
caught by stack check).
Blanket wide to all .c and .h files. Some files originating from ST are
difficult to deal with (license wise) so it was left out of those.
Also merged modpyb.h, modos.h, modstm.h and modtime.h in stmhal/.
