The ability to change the host is a frequently requested feature, so
explicitly document how it can be achieved using the existing code.

See issues #2121, #4385, #4622, #5122, #5536.

So that USB can work.

This commit improves pllvalues.py to generate PLL values for H7 MCUs that
are valid (VCO in and out are in range) and extend for the entire range of
SYSCLK values up to 400MHz (up to 480MHz is currently unsupported).

It doesn't exist on these MCUs.

The cyw43 driver on stm32 will now work with H7 MCUs.

Because if the SYSCLK is set to 180MHz or higher it will require this to be
on already.

This commit implements a more complete replication of CPython's behaviour
for equality and inequality testing of objects.  This addresses the issues
discussed in #5382 and a few other inconsistencies.  Improvements over the
old code include:

- Support for returning non-boolean results from comparisons (as used by
  numpy and others).
- Support for non-reflexive equality tests.
- Preferential use of __ne__ methods and MP_BINARY_OP_NOT_EQUAL binary
  operators for inequality tests, when available.
- Fallback to op2 == op1 or op2 != op1 when op1 does not implement the
  (in)equality operators.

The scheme here makes use of a new flag, MP_TYPE_FLAG_NEEDS_FULL_EQ_TEST,
in the flags word of mp_obj_type_t to indicate if various shortcuts can or
cannot be used when performing equality and inequality tests.  Currently
four built-in classes have the flag set: float and complex are
non-reflexive (since nan != nan) while bytearray and frozenszet instances
can equal other builtin class instances (bytes and set respectively).  The
flag is also set for any new class defined by the user.

This commit also includes a more comprehensive set of tests for the
behaviour of (in)equality operators implemented in special methods.

This board now has the following 3 build configurations:
- mboot + external QSPI in XIP mode + internal filesystem
- mboot + external QSPI with filesystem (the default)
- no mboot + external QSPI with filesystem

With a SPI flash that has more than 16MB, 32-bit addressing is required
rather than the standard 24-bit.  This commit adds support for 32-bit
addressing so that the SPI flash commands (read/write/erase) are selected
automatically depending on the size of the address being used at each
operation.

This modifies the signature of mp_thread_set_state() to use
mp_state_thread_t* instead of void*.  This matches the return type of
mp_thread_get_state(), which returns the same value.

`struct _mp_state_thread_t;` had to be moved before
`#include <mpthreadport.h>` since the stm32 port uses it in its
mpthreadport.h file.

PLLM is shared among all PLL blocks on F7 MCUs, and this calculation to
configure PLLSAI to have 48MHz on the P output previously assumed that PLLM
is equal to HSE (eg PLLM=25 for HSE=25MHz).  This commit relaxes this
assumption to allow other values of PLLM.

The loop searches backwards for a target, but doesn't stop after finding
the first result, meaning that it'll always end up at the outermost
exception handler.

This previously made the native emitter incompatible with the bytecode
emitter, and mp_resume (and subsequently mp_obj_generator_resume) expects
the bytecode emitter behavior (i.e. throw==NULL).

This commit adds a generator test for throwing into a nested exception, and
one when using yield-from with a pending exception cleanup.  Both these
tests currently fail on the native emitter, and are simplified versions of
native test failures from uasyncio in #5332.

It is not safe to enable MICROPY_ASYNC_KBD_INTR and MICROPY_PY_THREAD_GIL
at the same time.  This will trigger a compiler error to ensure that it
is not possible to make this mistake.

This releases the GIL during syscalls that could block.

This releases the GIL during POSIX system calls that could block.

This releases the GIL during syscalls that could block.

Addition of GIL EXIT/ENTER pairs are:

- modos: release the GIL during system calls.  CPython does this as well.

- moduselect: release the GIL during the poll() syscall.  This call can be
  blocking, so it is important to allow other threads to run at this time.

- modusocket: release the GIL during system calls.  Many of these calls can
  be blocking, so it is important to allow other threads to run.

- unix_mphal: release the GIL during the read and write syscalls in
  mp_hal_stdin_rx_chr and mp_hal_stdout_tx_strn.  If we don't do this
  threads are blocked when the REPL or the builtin input function are used.

- file, main, mpconfigport.h: release GIL during syscalls in built-in
  functions that could block.

When CFLAGS_EXTRA/LDFLAGS_EXTRA (or anything) is set on the command line of
a make invocation then it will completely override any setting or appending
of these variables in the makefile(s).  This means builds like the coverage
variant will have their mpconfigvariant.mk settings overridden.  Fix this
by using CFLAGS/LDFLAGS exclusively in the makefile(s), reserving the
CFLAGS_EXTRA/LDFLAGS_EXTRA variables for external command-line use only.

As suggested by @dpgeorge in #5538.

Commit d96cfd13 introduced a regression in
testing for bool objects, that such objects were in some cases no longer
recognised and bools, eg when using mp_obj_is_type(o, &mp_type_bool), or
mp_obj_is_integer(o).

This commit fixes that problem by adding mp_obj_is_bool(o).  Builds with
MICROPY_OBJ_IMMEDIATE_OBJS enabled check if the object is any of the const
True or False objects.  Builds without it use the old method of ->type
checking, which compiles to smaller code (compared with the former
mentioned method).

Fixes #5538.

Because it's no longer called anywhere in the code.

This reduces code size by 10-70 bytes on all ports (except cc3200 which has
no change).

When threads and the GIL are enabled, then the qstr mutex is not needed.
The qstr_mutex field is never used in this case because of:

    #if MICROPY_PY_THREAD && !MICROPY_PY_THREAD_GIL
    #define QSTR_ENTER() mp_thread_mutex_lock(&MP_STATE_VM(qstr_mutex), 1)
    #define QSTR_EXIT() mp_thread_mutex_unlock(&MP_STATE_VM(qstr_mutex))
    #else
    #define QSTR_ENTER()
    #define QSTR_EXIT()
    #endif

So, we can completely remove qstr_mutex everywhere when MICROPY_PY_THREAD
&& !MICROPY_PY_THREAD_GIL.

When threads and the GIL are enabled, then the GC mutex is not needed.  The
gc_mutex field is never used in this case because of:

    #if MICROPY_PY_THREAD && !MICROPY_PY_THREAD_GIL
    #define GC_ENTER() mp_thread_mutex_lock(&MP_STATE_MEM(gc_mutex), 1)
    #define GC_EXIT() mp_thread_mutex_unlock(&MP_STATE_MEM(gc_mutex))
    #else
    #define GC_ENTER()
    #define GC_EXIT()
    #endif

So, we can completely remove gc_mutex everywhere when MICROPY_PY_THREAD
&& !MICROPY_PY_THREAD_GIL.

Some parts of code have been aligned to increase readability.  In general
'' instead of "" were used wherever possible to keep the same convention
for entire file.  Import inspect line has been moved to the top according
to hints reported by pep8 tools.  A few extra spaces were removed, a few
missing spaces were added.  Comments have been updated, mostly in
"read_dfu_file" function.  Some other comments have been capitalized and/or
slightly updated.  A few docstrings were fixed as well.  No real code
changes intended.