Prior to this patch a negative duty would lead to full PWM.

This feature is not often used so is guarded by the config option
MICROPY_PY_BUILTINS_RANGE_BINOP which is disabled by default.  With this
option disabled MicroPython will always return false when comparing two
range objects for equality (unless they are exactly the same object
instance).  This does not match CPython so if (in)equality between range
objects is needed then this option should be enabled.

Enabling this option costs between 100 and 200 bytes of code space
depending on the machine architecture.

Most embedded targets will have this bit of the code disabled, saving a
small amount of code space.

This patch provides inline versions of the utf8 helper functions for the
case when unicode is disabled (MICROPY_PY_BUILTINS_STR_UNICODE set to 0).
This saves code size.

The unichar_charlen function is also renamed to utf8_charlen to match the
other utf8 helper functions, and the signature of this function is adjusted
for consistency (const char* -> const byte*, mp_uint_t -> size_t).

Instead of putting just 'CRASH' in the .py.out file, this patch makes it so
any output from uPy that led to the crash is stored in the .py.out file, as
well as the 'CRASH' message at the end.

For builds without the compiler enabled (and hence without eval/exec) it is
useful to still be able to run as many tests as possible.

Prior to this patch, storage.c was a combination of code that handled
either internal flash or external SPI flash and exposed one of them as a
block device for the local storage.  It was also exposed to the USB MSC.

This patch splits out the flash and SPI code to separate files, which each
provide a general block-device interface (at the C level).  Then storage.c
just picks one of them to use as the local storage medium.  The aim of this
factoring is to allow to add new block devices in the future and allow for
easier configurability.

This patch allows to completely compile-out support for USB, and no-USB is
now the default.  If a board wants to enable USB it should define:

    #define MICROPY_HW_ENABLE_USB (1)

And then one or more of the following to select the USB PHY:

    #define MICROPY_HW_USB_FS (1)
    #define MICROPY_HW_USB_HS (1)
    #define MICROPY_HW_USB_HS_IN_FS (1)

Newer versions of the HAL use names which are cleaner and more
self-consistent amongst the HAL itself.  This patch switches to use those
names in most places so it is easier to update the HAL in the future.

Prior to this patch the USBD driver did not handle the recipient correctly
for setup requests.  It was not interpreting the req->wIndex field in the
right way: in some cases this field indicates the endpoint number but the
code was assuming it always indicated the interface number.

This patch fixes this.  The only noticeable change is to the MSC
interface, which should now correctly respond to the USB_REQ_CLEAR_FEATURE
request and hence unmount properly from the host when requested.

mpconfigboard_common.h now sets the defaults so there is no longer a need
to explicitly list all configuration options in a board's mpconfigboard.h
file.

This file mirrors py/mpconfig.h but for board-level config options.  It
provides a default configuration, to be overridden by a specific
mpconfigboard.h file, as well as setting up certain macros to automatically
configure a board.

It was raising an exception but it should return infinity.

Prior to this patch, a float literal that was close to subnormal would
have a loss of precision when parsed.  The worst case was something like
float('10000000000000000000e-326') which returned 0.0.

This patch simplifies how sentinel values are stored on the stack when
doing an unwind return or jump.  Instead of storing two values on the stack
for an unwind jump it now stores only one: a negative small integer means
unwind-return and a non-negative small integer means unwind-jump with the
value being the number of exceptions to unwind.  The savings in code size
are:

   bare-arm:   -56
minimal x86:   -68
   unix x64:   -80
unix nanbox:    -4
      stm32:   -56
     cc3200:   -64
    esp8266:   -76
      esp32:  -156

The array should be of type unsigned byte because that is the type of the
values being stored.  And changing to uint8_t helps to prevent warnings
from some static analysers.

For consistency with all of the other tests that are named builtin_XXX.py.

Note that the check for elem!=NULL is removed for the
MP_MAP_LOOKUP_ADD_IF_NOT_FOUND case because mp_map_lookup will always
return non-NULL for such a case.

The calls to rtc_init_start(), sdcard_init() and storage_init() are all
guarded by a check for first_soft_reset, so it's simpler to just put them
all before the soft-reset loop, without the check.

The call to machine_init() can also go before the soft-reset loop because
it is only needed to check the reset cause which can happen once at the
first boot.  To allow this to work, the reset cause must be set to SOFT
upon a soft-reset, which is the role of the new function machine_deinit().

To use the LSE bypass feature (where an external source provides the RTC
clock) a board must set the config variable MICROPY_HW_RTC_USE_BYPASS.

Upon boot the RTC early-init function should detect if LSE or LSI is
already selected/running and, if so, use it.  When the LSI has previously
(in the previous reset cycle) been selected as the clock source the only
way to reliably tell is if the RTCSEL bits of the RCC_BDCR are set to the
correct LSI value.  In particular the RCC_CSR bits for LSI control do not
indicate if the LSI is ready even if it is selected.

This patch removes the check on the RCC_CSR bits for the LSI being on and
ready and only uses the check on the RCC_BDCR to see if the LSI should be
used straightaway.  This was tested on a PYBLITEv1.0 and with the patch the
LSI persists correctly as the RTC source as long as the backup domain
remains powered.

Previously, if LSE is selected but fails and the RTC falls back to LSI,
then the rtc_info flags would incorrectly state that LSE is used.  This
patch fixes that by setting the bit in rtc_info only after the clock is
ready.

Also, change ValueError to TypeError if the argument to this function is
not of an SPI type.

There is an underlying hardware SPI driver (built on top of the STM HAL)
and then on top of this sits the legacy pyb.SPI class as well as the
machine.SPI class.  This patch improves the separation between these
layers, in particular decoupling machine.SPI from pyb.SPI.

This patch combines the compiler optimisation code for double and triple
tuple-to-tuple assignment, taking it from two separate if-blocks to one
combined if-block.  This can be done because the code for both of these
optimisations has a lot in common.  Combining them together reduces code
size for ports that have the triple-tuple optimisation enabled (and doesn't
change code size for ports that have it disabled).

The I2C sub-system is independent from the uPy state (eg the heap) and so
can safely persist across a soft reset.