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On the STM32F722 (at least, but STM32F767 is not affected) the CK48MSEL bit
must be deselected before PLLSAION is turned off, or else the 48MHz
peripherals (RNG, SDMMC, USB) may get stuck without a clock source.

In such "lock up" cases it seems that these peripherals are still being
clocked from the PLLSAI even though the CK48MSEL bit is turned off.  A hard
reset does not get them out of this stuck state.  Enabling the PLLSAI and
then disabling it does get them out.  A test case to see this is:

    import machine, pyb
    for i in range(100):
        machine.freq(122_000000)
        machine.freq(120_000000)
        print(i, [pyb.rng() for _ in range(4)])

On occasion the RNG will just return 0's, but will get fixed again on the
next loop (when PLLSAI is enabled by the change to a SYSCLK of 122MHz).

Fixes issue #4696.

Otherwise converting large floats to ints will fail (as seen by the
builtin_float_hash.py test).

Selectable via pyb.usb_mode('VCP+VCP').

The stm32 and nrf ports already had the behaviour that they would first
check if the script exists before executing it, and this patch makes all
other ports work the same way.  This helps when developing apps because
it's hard to tell (when unconditionally trying to execute the scripts) if
the resulting OSError at boot up comes from missing boot.py or main.py, or
from some other error.  And it's not really an error if these scripts don't
exist.

It will only execute the script if it can be stat'd and is a file.

Prior to this patch, when a lot of data was output by a running script
pyboard.py would try to capture all of this output into the "data"
variable, which would gradually slow down pyboard.py to the point where it
would have large CPU and memory usage (on the host) and potentially lose
data.

This patch fixes this problem by not accumulating the data in the case that
the data is not needed, which is when "data_consumer" is used.

This patch makes the DAC driver simpler and removes the need for the ST
HAL.  As part of it, new helper functions are added to the DMA driver,
which also use direct register access instead of the ST HAL.

Main changes to the DAC interface are:

- The DAC uPy object is no longer allocated dynamically on the heap,
  rather it's statically allocated and the same object is retrieved for
  subsequent uses of pyb.DAC(<id>).  This allows to access the DAC objects
  without resetting the DAC peripheral.  It also means that the DAC is only
  reset if explicitly passed initialisation parameters, like "bits" or
  "buffering".

- The DAC.noise() and DAC.triangle() methods now output a signal which is
  full scale (previously it was a fraction of the full output voltage).

- The DAC.write_timed() method is fixed so that it continues in the
  background when another peripheral (eg SPI) uses the DMA (previously the
  DAC would stop if another peripheral finished with the DMA and shut the
  DMA peripheral off completely).

Based on the above, the following backwards incompatibilities are
introduced:

- pyb.DAC(id) will now only reset the DAC the first time it is called,
  whereas previously each call to create a DAC object would reset the DAC.
  To get the old behaviour pass the bits parameter like: pyb.DAC(id, bits).

- DAC.noise() and DAC.triangle() are now full scale.  To get previous
  behaviour (to change the amplitude and offset) write to the DAC_CR (MAMP
  bits) and DAC_DHR12Rx registers manually.

Fixes issue #4702.

To configure the SYSCLK on an F0 enable one of:

    MICROPY_HW_CLK_USE_HSI48
    MICROPY_HW_CLK_USE_HSE
    MICROPY_HW_CLK_USE_BYPASS

To be consistent with how F4/F7/H7/L4 works in system_stm32.c.  The power
control peripheral is needed at least for the RTC.

This adds tests for some locations in the code where a memory allocation
should raise an exception.

See issue #4655.

In CPython the random module is seeded differently on each import, and so
this new macro option MICROPY_PY_URANDOM_SEED_INIT_FUNC allows to implement
such a behaviour.

MODULE_EXAMPLE_ENABLED must be globally defined for the module to be seen
and referenced by all parts of the code.

Fixes issue #4693.

The issue described in the comment added here can be seen by forcing a
gc_collect() at the start of each call to gc_alloc().

In case (user) source code contains utf-8 encoded data and the default
locale is not utf-8.

See #4592.

And fix a typo in the comment on this line.

If MICROPY_HW_RTC_USE_BYPASS is enabled the RTC startup goes as follows:
- RTC is started with LSE in bypass mode to begin with
- if that fails to start (after a given timeout) then LSE is reconfigured
  in non-bypass
- if that fails to start then RTC is switched to LSI

To save space, since this board only hase 256k of flash.

The qstr window size is not log-2 encoded, it's just the actual number (but
in mpy-tool.py this didn't lead to an error because the size is just used
to truncate the window so it doesn't grow arbitrarily large in memory).

Addresses issue #4635.

Fixes the regression introduced in ea3c80a5