Calling it from mp_init() is too late for some ports (like Unix), and leads
to incomplete stack frame being captured, with following GC issues. So, now
each port should call mp_stack_ctrl_init() on its own, ASAP after startup,
and taking special precautions so it really was called before stack variables
get allocated (because if such variable with a pointer is missed, it may lead
to over-collecting (typical symptom is segfaulting)).

Currently nlr_jump_fail prints that there was an uncaught exception
but nothing about the exception.
This patch causes nlr_jump_failed to try to print the exception.
Given that printf was called on the line above, I think that
the call to mp_obj_print_exception has about as much likelyhood
of succeeding as the printf does.

When you use the USER button to perform a filesystem reset
at boot time then it wipes out the filesystem and creates
a new boot.py and main.py.  With this patch these files are
executed after formatting, ensuring that pyb and machine modules
get imported.

This is a hack to free up TIM3 so that it can be used by the user.
Instead we use the PVD irq to call the USB VCP polling function, and
trigger it from SysTick (so SysTick itself does not do any processing).

The feature is enabled for pyboard lite only, since it lacks timers.

If RTC is already running at boot then it's left alone.  Otherwise, RTC is
started at boot but startup function returns straight away.  RTC startup
is then finished the first time it is used.  Fallback to LSI if LSE fails
to start in a certain time.

Also included:
 MICROPY_HW_CLK_LAST_FREQ
        hold pyb.freq() parameters in RTC backup reg
 MICROPY_HW_RTC_USE_US
        option to present datetime sub-seconds in microseconds
 MICROPY_HW_RTC_USE_CALOUT
        option to enable RTC calibration output

CLK_LAST_FREQ and RTC_USE_CALOUT are enabled for PYBv1.0.

py/mphal.h contains declarations for generic mp_hal_XXX functions, such
as stdio and delay/ticks, which ports should provide definitions for.  A
port will also provide mphalport.h with further HAL declarations.

Use this to set the global optimisation value when executing the main
script (and all scripts it imports).

Thanks to Dave Hylands for the patch.

This helps make files reusable across other ports.

Now all ports can use pybstdio.c to provide sys.stdin/stdout/stderr, so
long as they implement mp_hal_stdin_* and mp_hal_stdout_* functions.

Different HID modes can be configured in Python.  You can either use
predefined mouse or keyboard, or write your own report descriptor.

A GC in stmhal port now only scans true root pointers, not entire BSS.
This reduces base GC time from 1700ms to 900ms.

This is so it's compatible with Windows.

UART object now uses a stream-like interface: read, readall, readline,
readinto, readchar, write, writechar.

Timeouts are configured when the UART object is initialised, using
timeout and timeout_char keyword args.

The object includes optional read buffering, using interrupts.  You can set
the buffer size dynamically using read_buf_len keyword arg.  A size of 0
disables buffering.

As per issue #876, the network module is used to configure NICs
(hardware modules) and configure routing.  The usocket module is
supposed to implement the normal Python socket module and selects the
underlying NIC using routing logic.

Right now the routing logic is brain dead: first-initialised,
first-used.  And the routing table is just a list of registered NICs.

cc3k and wiznet5k work, but not at the same time due to C name clashes
(to be fixed).

Note that the usocket module has alias socket, so that one can import
socket and it works as normal.  But you can also override socket with
your own module, using usocket at the backend.

Stack is full descending and must be 8-byte aligned.  It must start off
pointing to just above the last byte of RAM.

Previously, stack started pointed to last byte of RAM (eg 0x2001ffff)
and so was not 8-byte aligned.  This caused a bug in combination with
alloca.

This patch also updates some debug printing code.

Addresses issue #872 (among many other undiscovered issues).

sys.exit always raises SystemExit so doesn't need a special
implementation for each port.  If C exit() is really needed, use the
standard os._exit function.

Also initialise mp_sys_path and mp_sys_argv in teensy port.

Factored irq functions into a separate file.

It's still "safe" because no scripts are run.  Remove the SD card if you
want to access the internal flash filesystem.  Addresses issue #616.

Also: remove obsolete pyb.source_dir setting, and reset pyb.main and
pyb.usb_mode settings on soft-reset.

Make a clearer distinction between init functions that must be done
before any scripts can run (xxx_init0) and those that can be safely
deferred (xxx_init).

Fix bug initialising USB VCP exception.  Addresses issue #788.

Re-order some init function to improve reliability of
reset/soft-reset.

qstr_init is always called exactly before mp_init, so makes sense to
just have mp_init call it.  Similarly with
mp_init_emergency_exception_buf.  Doing this makes the ports simpler and
less error prone (ie they can no longer forget to call these).

Some important changes to the way the file system is structured on the
pyboard:

1. 0: and 1: drive names are now replaced with POSIX inspired
directories, namely /flash and /sd.

2. Filesystem now supports the notion of a current working directory.
Supports the standard Python way of manipulating it: os.chdir and
os.getcwd.

3. On boot up, current directory is /flash if no SD inserted, else /sd
if SD inserted.  Then runs boot.py and main.py from the current dir.
This is the same as the old behaviour, but is much more consistent and
flexible (eg you can os.chdir in boot.py to change where main.py is run
from).

4. sys.path (for import) is now set to '' (current dir), plus /flash
and /flash/lib, and then /sd and /sd/lib if SD inserted.  This, along
with CWD, means that import now works properly.  You can import a file
from the current directory.

5. os.listdir is fixed to return just the basename, not the full path.

See issue #537 for background and discussion.

The user code should call micropython.alloc_emergency_exception_buf(size)
where size is the size of the buffer used to print the argument
passed to the exception.

With the test code from #732, and a call to
micropython.alloc_emergenncy_exception_buf(100) the following error is
now printed:
```python
>>> import heartbeat_irq
Uncaught exception in Timer(4) interrupt handler
Traceback (most recent call last):
  File "0://heartbeat_irq.py", line 14, in heartbeat_cb
NameError: name 'led' is not defined
```