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Previously the hardware I2C timeout was hard coded to 50ms which isn't
guaranteed to be enough depending on the clock stretching specs of the I2C
device(s) in use.

This patch ensures the hardware I2C implementation honors the existing
timeout argument passed to the machine.I2C constructor.  The default
timeout for software and hardware I2C is now 50ms.

On stm32 boards, machine.I2C is now preferred over pyb.I2C.

Fixes issue #3482.

For example: i2c.writevto(addr, (buf1, buf2)).  This allows to efficiently
(wrt memory) write data composed of separate buffers, such as a command
followed by a large amount of data.

API is:

    int transfer(
        mp_obj_base_t *obj,
        uint16_t addr,
        size_t n,
        mp_machine_i2c_buf_t *bufs,
        unsigned int flags
    )

This allows to efficiently send to an I2C slave data that is made up of
more than one buffer.  Instead of needing to allocate temporary memory to
combine buffers together this new method allows to pass in a tuple or list
of buffers.  The name is based on the POSIX function writev() which has
similar intentions and signature.

The reasons for taking this approach (compared to having an interface with
separate start/write/stop methods) are:
- It's a backwards compatible extension.
- It's convenient for the user.
- It's efficient because there is only one Python call, then the C code can
  do everything in one go.
- It's efficient on the I2C bus because the implementation can do
  everything in one go without pauses between blocks of bytes.
- It should be possible to implement this extension in all ports, for
  hardware and software I2C.

Further discussion is found in issue #3482, PR #4020 and PR #4763.

Fixes issue #4794.

For modules I initially created or made substantial contributions to.

Recent gcc versions (at least 9.1) give a warning about using "sp" in the
clobber list.  Such code is removed by this patch.  A dedicated function is
instead used to set SP and branch to the bootloader so the code has full
control over what happens.

Fixes issue #4785.

This also fixes deleting the PPP task, since eTaskGetState() never returns
eDeleted.

A limitation with this patch: once the PPP is deactivated (ppp.active(0))
it cannot be used again. A new PPP instance must be created instead.

Only the M4 and M7 MCUs have an FPU and FPU_IRQn, and FPU_IRQn is not
always the last entry/IRQ number.

The H7 HAL uses SPI IRQs when the SPI is running in DMA mode.

This is required for the H7 DMA to work.

The user can now select their own package index by either passing the "-i"
command line option, or setting the upip.index_urls variable (before doing
an install).

The https://micropython.org/pi package index hosts packages from
micropython-lib and will be searched first when installing a package.  If a
package is not found here then it will fallback to PyPI.

This allows figuring out the number of bytes in the memoryview object as
len(memview) * memview.itemsize.

The feature is enabled via MICROPY_PY_BUILTINS_MEMORYVIEW_ITEMSIZE and is
disabled by default.

Follow up to commit 34942d0a

Fixes issue #4780.

Can now handle up to about 298 days maximum for millisecond periods.

Fixes issue #4664.

Macro identifiers with a leading underscore are reserved.

Fixes issue #4527.

The original code called setsockopt(SO_RCVTIMEO/SO_SNDTIMEO) with NULL
timeout structure argument, which is an illegal usage of that function.
The old code also didn't validate the return value of setsockopt, missing
the bug completely.

Before this change, if the USB was reconnected it was possible that some
characters in the TX buffer were retransmitted because tx_buf_ptr_out and
tx_buf_ptr_out_shadow were reset while tx_buf_ptr_in wasn't.  That
behaviour is fixed here by retaining the TX buffer state across reconnects.

Fixes issue #4761.

The new function factory_reset_make_files() populates the given filesystem
with the default factory files.  It is defined with weak linkage so it can
be overridden by a board.

This commit also brings some minor user-facing changes:

- boot.py is now no longer created unconditionally if it doesn't exist, it
  is now only created when the filesystem is formatted and the other files
  are populated (so, before, if the user deleted boot.py it would be
  recreated at next boot; now it won't be).

- pybcdc.inf and README.txt are only created if the board has USB, because
  they only really make sense if the filesystem is exposed via USB.

It's more common to need non-blocking behaviour when reading from a UART,
rather than having a large timeout like 1000ms (the original behaviour).
With a large timeout it's 1) likely that the function will read forever if
characters keep trickling it; or 2) the function will unnecessarily wait
when characters come sporadically, eg at a REPL prompt.