Default is enabled, disabled for minimal builds. Saves 1296 bytes on x86,
976 bytes on ARM.

In 0e80f345 the inplace operations __iadd__
and __isub__ were made unconditionally available, so the comment about this
section is changed to reflect that.

The intention of oflush() is to flush the "fast SPI" command itself so that
the SPI object is ready to use when the function returns.

Changes made:
- fix DMA_SUB_INSTANCE_AS_UINT8
- fix dma_id numbers in dma_descr_t
- add F0 DMA IRQ handlers
- set DmaBaseAddress and ChannelIndex when reinit'ing

Fixes issue #4113.

Loading a pointer by indexing into the native function table mp_fun_table,
rather than loading an immediate value (via a PC-relative load), uses less
code space.

Viper functions will now capture the globals at the point they were defined
and use these globals when executing.

Old globals are now stored in the second slot (ip in mp_code_state_t) to
make things simpler for viper.

This commit makes viper functions have the same signature as native
functions, at the level of the emitter/assembler.  This means that viper
functions can now be wrapped in the same uPy object as native functions.

Viper functions are now responsible for parsing their arguments (before it
was done by the runtime), and this makes calling them more efficient (in
most cases) because the viper entry code can be custom generated to suit
the signature of the function.

This change also opens the way forward for viper functions to take
arbitrary numbers of arguments, and for them to handle globals correctly,
among other things.

Now that the compiler can store the results of the viper types in the
scope, the viper parameter annotation compilation stage can be merged with
the normal parameter compilation stage.

In viper mode, the type of the argument is now stored in id_info->flags.

Instead this return type is now stored in the scope_flags.

The native emitter can easily determine the mode via scope->emit_options.

With 5 arguments to mp_arg_check_num(), some architectures need to pass
values on the stack.  So compressing n_args_min, n_args_max, takes_kw into
a single word and passing only 3 arguments makes the call more efficient,
because almost all calls to this function pass in constant values.  Code
size is also reduced by a decent amount:

   bare-arm:  -116
minimal x86:   -64
   unix x64:  -256
unix nanbox:  -112
      stm32:  -324
     cc3200:  -192
    esp8266:  -192
      esp32:  -144

This issue was brought up by BramPeters in the forum:
https://forum.micropython.org/viewtopic.php?p=30066

If DTTOIF() macro is not defined, the code refers to MP_S_IFDIR, etc.
symbols defined in extmod/vfs.h, so should include it.

This fixes build for Android.

Added cmake_minimum_required and updated target_link_libraries directives.

net_config subsystem was split off from net_app, and as a result, settings
need renaming from CONFIG_NET_APP_* to CONFIG_NET_CONFIG_*.

Prior to this commit a function compiled with the native decorator
@micropython.native would not work correctly when accessing global
variables, because the globals dict was not being set upon function entry.

This commit fixes this problem by, upon function entry, setting as the
current globals dict the globals dict context the function was defined
within, as per normal Python semantics, and as bytecode does.  Upon
function exit the original globals dict is restored.

In order to restore the globals dict when an exception is raised the native
function must guard its internals with an nlr_push/nlr_pop pair.  Because
this push/pop is relatively expensive, in both C stack usage for the
nlr_buf_t and CPU execution time, the implementation here optimises things
as much as possible.  First, the compiler keeps track of whether a function
even needs to access global variables.  Using this information the native
emitter then generates three different kinds of code:

1. no globals used, no exception handlers: no nlr handling code and no
   setting of the globals dict.

2. globals used, no exception handlers: an nlr_buf_t is allocated on the
   C stack but it is not used if the globals dict is unchanged, saving
   execution time because nlr_push/nlr_pop don't need to run.

3. function has exception handlers, may use globals: an nlr_buf_t is
   allocated and nlr_push/nlr_pop are always called.

In the end, native functions that don't access globals and don't have
exception handlers will run more efficiently than those that do.

Fixes issue #1573.

The HAL DMA functions enable SDMMC interrupts before fully resetting the
peripheral, and this can lead to a DTIMEOUT IRQ during the initialisation
of the DMA transfer, which then clears out the DMA state and leads to the
read/write not working at all.  The DTIMEOUT is there from previous SDMMC
DMA transfers, even those that succeeded, and is of duration ~180 seconds,
which is 0xffffffff / 24MHz (default DTIMER value, and clock of
peripheral).

To work around this issue, fully reset the SDMMC peripheral before calling
the HAL SD DMA functions.

Fixes issue #4110.

Since mbedtls 2.7.0 new digest functions were introduced with a "_ret"
suffix to allow the functions to return an error message (eg, if the
underlying hardware acceleration failed).  These new functions must be used
instead of the old ones to prevent deprecation warnings, or link errors for
missing functions, depending on the mbedtls configuration.

The flash-IRQ handler is used to flush the storage cache, ie write
outstanding block data from RAM to flash.  This is triggered by a timeout,
or by a direct call to flush all storage caches.

Prior to this commit, a timeout could trigger the cache flushing to occur
during the execution of a read/write to external SPI flash storage.  In
such a case the storage subsystem would break down.

SPI storage transfers are already protected against USB IRQs, so by
changing the priority of the flash IRQ to that of the USB IRQ (what is
done in this commit) the SPI transfers can be protected against any
timeouts triggering a cache flush (the cache flush would be postponed until
after the transfer finished, but note that in the case of SPI writes the
timeout is rescheduled after the transfer finishes).

The handling of internal flash sync'ing needs to be changed to directly
call flash_bdev_irq_handler() sync may be called with the IRQ priority
already raised (eg when called from a USB MSC IRQ handler).

No need to be wasteful on DMA resources.