This is a more consistent use of errno codes.  For example, it may be that
a stream returns MP_EAGAIN but the mp_is_nonblocking_error() macro doesn't
catch this value because it checks for EAGAIN instead (which may have a
different value than MP_EAGAIN when MICROPY_USE_INTERNAL_ERRNO is enabled).

Most modern systems have EWOULDBLOCK aliased to EAGAIN, ie they have the
same value.  But some systems use different values for these errnos and if
a uPy port is using the system errno values (ie not the internal uPy
values) then it's important to be able to distinguish EWOULDBLOCK from
EAGAIN.  Eg if a system call returned EWOULDBLOCK it must be possible to
check for this return value, and this patch makes this now possible.

If MICROPY_USE_INTERNAL_ERRNO is disabled, MP_EINVAL is not guaranteed
to have the value 22, so we cannot depend on OSError(22,).
Instead, to support any given port's errno values, without relying
on uerrno, we just check that the args[0] is positive.

ADC3 is used because the H7's internal ADC channels are connected to ADC3
and the uPy driver doesn't support more than one ADC.

Only 12-bit resolution is supported because 12 is hard-coded and 14/16 bits
are not recommended on some ADC3 pins (see errata).

Values from internal ADC channels are known to give wrong values at
present.

WebREPL now works on the esp32 in the same way it does on esp8266.

The esp8266 uses modlwip.c for its usocket implementation, which allows to
easily support callbacks on socket events (like when a socket becomes ready
for reading).  This is not as easy to do for the esp32 which uses the
ESP-IDF-provided lwIP POSIX socket API.  Socket events are needed to get
WebREPL working, and this patch provides a way for such events to work by
explicitly polling registered sockets for readability, and then calling the
associated callback if the socket is readable.

This makes way for enabling uos.dupterm().

This follows how all other boards are configured.

Clang defines __GNUC__ so we have to check for it specifically.

After calling HAL_SYSTICK_Config the SysTick IRQ priority is set to 15, the
lowest priority.  This commit reconfigures the IRQ priority to the desired
TICK_INT_PRIORITY value.

Thanks to @bboser for the initial idea and implementation.

It's more efficient and improves accuracy.

i2c.c implements the legacy pyb.I2C class so rename the file to make this
explicit, and also to make room for an improved I2C driver.

By default the stm module is included in the build, but a board can now
define MICROPY_PY_STM to 0 to not include this module.  This reduces the
firmware by about 7k.

In particular don't issue a warning if the passed-in constraints are
actually supported because they are the default values.

Uses new pypi.org URL, and now creates a socket with the address parameters
returned by getaddrinfo().

To change the default label a board should define:

    #define MICROPY_HW_FLASH_FS_LABEL "label"

To use HSE bypass mode the board should define:

    #define MICROPY_HW_CLK_USE_BYPASS (1)

If this is not defined, or is defined to 0, then HSE oscillator mode is
used.

This patch allows a given board to configure which pins are used for the
CAN peripherals, in a similar way to all the other bus peripherals (I2C,
UART, SPI).  To enable CAN on a board the mpconfigboard.h file should
define (for example):

    #define MICROPY_HW_CAN1_TX (pin_B9)
    #define MICROPY_HW_CAN1_RX (pin_B8)
    #define MICROPY_HW_CAN2_TX (pin_B13)
    #define MICROPY_HW_CAN2_RX (pin_B12)

And the board config file should no longer define MICROPY_HW_ENABLE_CAN.

The individual union members (like SPI, I2C) are never used, only the
generic "reg" entry is.  And the union names can clash with macro
definitions in the HAL so better to remove them.

The only configuration that changes with this patch is that on L4 MCUs the
clock prescaler changed from ADC_CLOCK_ASYNC_DIV2 to ADC_CLOCK_ASYNC_DIV1
for the ADCAll object.  This should be ok.

A value of DISABLE for EOCSelection is invalid.  This would have been
interpreted instead as ADC_EOC_SEQ_CONV, but really it should be
ADC_EOC_SINGLE_CONV for the uses in this code.  So this has been fixed.

ExternalTrigConv should be ADC_SOFTWARE_START because all ADC
conversions are started by software.  This is now fixed.