Usage:

    import machine
    wdt = machine.WDT(0, 5000) # 5 second timeout
    wdt.feed()

Thanks to Moritz for the initial implementation.

This extra forward slash for the starting-point directory is unnecessary
and leads to additional slashes on Max OS X which mean that the frozen
files cannot be imported.

Fixes #2374.

This patch makes second and next calls to <socket>.close() a no-op.
It prevents GC from closing the underlying resource after user
already used <socket>.close() explicitly.

fixes #2355

This updates the print function for machine.Pin to
print out the new constants from the Hardware API defintion
rather than the legacy definitions.

TIM3 is no longer special, or at least does not have special IRQ settings.

It turns out that TIM1 and TIM8 have their own Capture/Compare
interrupt vector. For all of the other timers, the capture/compare
interrupt vector is the same as the update vector.

So we need to add handlers for these vectors and enable them
when using capture/compare callbacks.

During testing of this, I also found that passing a channel callback
into the channel constructor would not enable interrupts properly.

I tested using:
```
>>> pyb.Timer(1, freq=4).channel(1, pyb.Timer.OC_TOGGLE, callback=lambda t: print('.', end=''))
```
I tested the above with channels 1, 4, and 8

This type was used only for the typedef of mp_obj_t, which is now defined
by the object representation.  So we can now remove this unused typedef,
to simplify the mpconfigport.h file.

Fixing Issue #2243. Main problems were:

- HAL_ADC_GetState(adcHandle) may return other bits set (not only
  HAL_ADC_STATE_EOC_REG) when called - so I AND-ed it out as proposed by
  mattbrejza in Issue #2243.
- ADC Pin has to be configured as GPIO_MODE_ANALOG_ADC_CONTROL not only
  GPIO_MODE_ANALOG.
- Resolved ADC resolution L4 specific (Use L4 define ADC_RESOLUTION_12B).
- Changed setting of Init.EOCSelection toADC_EOC_SINGLE_CONV for L4.
- Added call to ADC_MultiModeTypeDef as this is done on a STM32Cube
  generated project too.
- Clean up: Configuration of ADC is done only in ONE function not the same
  is done in two functions.

Test is done on PA5 pin of STM32L4Discovery-Kit which is connected to the
DOWN button.

Thanks to mattbrejza for discovering the bug.

This patch introduces proper ADC Pin definitions in stm32l476_af.csv.
Originally provided by @tobbad.

Some boards (like the GHI Electronics G30 Dev Board) don't use
NSS at all and rather just use GPIO chip selects.

It's now used for more than just stream protocol (e.g. pin protocol), so
don't use false names.

Argument types were converted, but old function call of
mp_obj_new_exception_msg_varg() remained.

mp_hal_pin_config_od is renamed mp_hal_pin_open_drain, and mp_hal_pin_low
is mp_hal_pin_od_low.

See issue #1814.

For example, the following code now works with a file on the SD card:

    f = open('test', 'rb') # test must be 1024 bytes or more in size
    f.seek(511)
    f.read(513)

Also works for writing.

Fixes issue #1863.

If custom C code uses the I2C busses then it needs access to these
structures for i2c_init().

This allows PYBv3 to use PWM for LED(1) and LED(2).

Initialisation of CAN objects should now behave as other peripheral
objects.

Fixes issue #2001.

Addresses issue #2034.

Also raise an exception for machine.freq and machine.deepsleep on this
MCU, since they are not yet implemented.