On x86 archs (both 32 and 64 bit) a bool return value only sets the 8-bit
al register, and the higher bits of the ax register have an undefined
value.  When testing the return value of such cases it is required to just
test al for zero/non-zero.  On the other hand, checking for truth or
zero/non-zero on an integer return value requires checking all bits of the
register.  These two cases must be distinguished and handled correctly in
generated native code.  This patch makes sure of this.

For other supported native archs (ARM, Thumb2, Xtensa) there is no such
distinction and this patch does not change anything for them.

The Python documentation recommends to pass the command as a string when
using Popen(..., shell=True).  This is because "sh -c <string>" is used to
execute the command and additional arguments after the command string are
passed to the shell itself (not the executing command).

https://docs.python.org/3.5/library/subprocess.html#subprocess.Popen

And increase sampling time to get better results for internal channels.

Prior to this patch, if VBAT was read via ADC.read() or
ADCAll.read_channel(), then it would remain enabled and subsequent reads
of TEMPSENSOR or VREFINT would not work.  This patch makes sure that VBAT
is disabled for all cases that it could be read.

Also, clean up some code.
Code size change:
nrf51: -24
nrf52: -28

The hard_configs table has entries for each PWM instance. Use them.

DEBUG_printf and MICROPY_DEBUG_PRINTER is now used instead of normal
printf, and a fault is fixed in mp_obj_class_lookup with debugging enabled;
see issue #3999.  Debugging can now be enabled on all ports including when
nan-boxing is used.

This patch in effect renames MICROPY_DEBUG_PRINTER_DEST to
MICROPY_DEBUG_PRINTER, moving its default definition from
lib/utils/printf.c to py/mpconfig.h to make it official and documented, and
makes this macro a pointer rather than the actual mp_print_t struct.  This
is done to get consistency with MICROPY_ERROR_PRINTER, and provide this
macro for use outside just lib/utils/printf.c.

Ports are updated to use the new macro name.

Avoid trying to use ADC, PWM and RTCounter instances which is
one past last available, because this will give a HardFault.

The NRF52 define only covers nrf52832, so update the define checks
to use NRF52_SERIES to cover both nrf52832 and nrf52840.

Fixed machine_hard_pwm_instances table in modules/machine/pwm.c

This enables PWM(0) to PWM(3), RTCounter(2), Timer(3) and Timer(4),
in addition to NFC reset cause, on nrf52840.

This will make 'with open('file', 'r') as f:' work by properly close
the file after the suite is finished.

The first dynamic qstr pool is double the size of the 'alloc' field of
the last const qstr pool. The built in const qstr pool
(mp_qstr_const_pool) has a hardcoded alloc size of 10, meaning that the
first dynamic pool is allocated space for 20 entries. The alloc size
must be less than or equal to the actual number of qstrs in the pool
(the 'len' field) to ensure that the first dynamically created qstr
triggers the creation of a new pool.

When modules are frozen a second const pool is created (generally
mp_qstr_frozen_const_pool) and linked to the built in pool. However,
this second const pool had its 'alloc' field set to the number of qstrs
in the pool. When freezing a large quantity of modules this can result
in thousands of qstrs being in the pool. This means that the first
dynamically created qstr results in a massive allocation. This commit
sets the alloc size of the frozen qstr pool to 10 or less (if the number
of qstrs in the pool is less than 10). The result of this is that the
allocation behaviour when a dynamic qstr is created is identical with an
without frozen code.

Note that there is the potential for a slight memory inefficiency if the
frozen modules have less than 10 qstrs, as the first few dynamic
allocations will have quite a large overhead, but the geometric growth
soon deals with this.

When waking from stop mode most of the system is still in the same state as
before entering stop, so only minimal configuration is needed to bring the
system clock back online.

The WiPy machine.Timer class is very different to the esp8266 and esp32
implementations which are better candidates for a general Timer class.  By
moving the WiPy Timer docs to a completely separate file, under a new name
machine.TimerWiPy, it gives a clean slate to define and write the docs for
a better, general machine.Timer class.  This is with the aim of eventually
providing documentation that does not have conditional parts to it,
conditional on the port.

While the new docs are being defined it makes sense to keep the WiPy docs,
since they describe its behaviour.  Once the new Timer behaviour is defined
the WiPy code can be changed to match it, and then the TimerWiPy docs would
be removed.

The problem was that the EXTI line for the RTC wakeup event is line 20 on
the F0, so the interrupt was not firing.

This patch makes the Thumb-2 native emitter use wide ldr instructions to
call into the runtime, when the index into the native glue function table
is 32 or greater.  This reduces the generated assembler code from 10 bytes
to 6 bytes, saving RAM and making native code run about 0.8% faster.

A recent version of arm-none-eabi-gcc (8.2.0) will warn about unused packed
attributes in USB_WritePacket and USB_ReadPacket.  This patch suppresses
such warnings for this file only.

This error message did not consume all of its variable args, a bug
introduced long ago in baf6f14d.  By fixing
it to use %s (instead of keeping the string as-is and deleting the last
arg) the same error message string is now reused three times in this format
function and gives a code size reduction of around 130 bytes.  It also now
gives a better error message when a non-string is passed in as an argument
to format, eg '{:d}'.format([]).

Otherwise returning None indicates that the write would block and nothing
was actually written.  Fixes issue #3990.

This patch will work for both Python 2 and 3.

This prevents hard-faults on non-aligned accesses.

Reference: http://www.keil.com/support/docs/3777.htm

If SDRAM is configured and enabled for a board then it is used for the
MicroPython GC heap.

Taken from https://github.com/openmv/openmv/blob/7fbe54ad4e9dc1c527d9297a3f4c69c365dc9b2c/src/omv/sdram.c

Code is is MIT licensed.

Instead of downloading "a" version, these links point to history from where
you can download the verson you like.

The machine module should be standard across all ports so should have the
same set of classes in the docs.  A special warning is added to the top of
the machine.SD class because it is not standardised and only available on
the cc3200 port.

The heading of this section makes it clear it is for Thumb-2 architectures
only.

It's fair to just provide a link to all available modules, regardless of
the port.  Most of the existing ports (unix, stm32, esp8266, esp32) share
most of the same set of modules anyway, so no need to maintain separate
lists for them.  And there's a big discussion at the start of this index
about modules not being available on a given port.

For port-specific modules, they can also be listed unconditionally because
they have headings that explicitly state they are only available on certain
ports.

pyusb v1.0.2 warns about `getargspec` as being deprecated.

Thanks to @dhylands for this patch to pad the image to 8-byte boundaries.