Some DMA channels (eg for SDIO) can be used in both directions and this
patch allows such peripherals to dynamically select the DMA direction.

MCUs that have a PLLSAI can use it to generate a 48MHz clock for USB, SDIO
and RNG peripherals.  In such cases the SYSCLK is not restricted to values
that allow the system PLL to generate 48MHz, but can be any frequency.
This patch allows such configurability for F7 MCUs, allowing the SYSCLK to
be set in 2MHz increments via machine.freq().  PLLSAI will only be enabled
if needed, and consumes about 1mA extra.  This fine grained control of
frequency is useful to get accurate SPI baudrates, for example.

This makes it easy to add a custom board definition outside of the
micropython tree, keeping the micropython submodule clean and official.

If bytearray is constructed from str, a second argument of encoding is
required (in CPython), and third arg of Unicode error handling is allowed,
e.g.:

bytearray("str", "utf-8", "strict")

This is similar to bytes:

bytes("str", "utf-8", "strict")

This patch just allows to pass 2nd/3rd arguments to bytearray, but
doesn't try to validate them to not impact code size. (This is also
similar to how bytes constructor is handled, though it does a bit
more validation, e.g. check that in case of str arg, encoding argument
is passed.)

Based on tests/extmod/uhashlib_sha1.

MD5 is still widely used, and may be important in some cases for networking
interoperability, e.g. HTTP Digest authentication.

Avoids polluting the source tree, allows to build for different (sub)archs
without intermediate cleaning.

This removes the need for a separate axtls build stage, and builds all
axtls object files along with other code.  This simplifies and cleans up
the build process, automatically builds axtls when needed, and puts the
axtls object files in the correct $(BUILD) location.

The MicroPython axtls configuration file is provided in
extmod/axtls-include/config.h

To eliminate the need for symlinks which don't work on systems like
Windows.

As per ST's DfuSe specification, and following their example code.

A recent version of arm-none-eabi-gcc (8.2.0) will warn about unused packed
attributes in USB_WritePacket and USB_ReadPacket.  This patch suppresses
such warnings for this file only.

Prior to this patch tanhf(large number) would return nan due to inf/inf.

Prior to this patch tanh(large number) would return nan due to inf/inf.

This patch adds full support for unwinding jumps to the native emitter.
This means that return/break/continue can be used in try-except,
try-finally and with statements.  For code that doesn't use unwinding jumps
there is almost no overhead added to the generated code.

The native emitter keeps the current exception in a slot in its C stack
(instead of on its Python value stack), so when it catches an exception it
must explicitly clear that slot so the same exception is not reraised later
on.

Back in 8047340d basic support was added in
the VM to handle return statements within a finally block.  But it didn't
cover all cases, in particular when some finally's were active and others
inactive when the "return" was executed.

This patch adds further support for return-within-finally by correctly
managing the currently_in_except_block flag, and should fix all cases.  The
main point is that finally handlers remain on the exception stack even if
they are active (currently being executed), and the unwind return code
should only execute those finally's which are inactive.

New tests are added for the cases which now pass.

PEP479 (see https://www.python.org/dev/peps/pep-0479/) prohibited raising
StopIteration from within a generator (it is turned into a RuntimeError).
This behaviour was introduced in Python 3.5 and in 3.7 was made compulsory.
Until uPy implements PEP479, this patch adds .py.exp files for the relevant
tests so they can be run under Python 3.7.

In Python 3.7 the behaviour of repr() of an exception with one argument
changed: it no longer prints a trailing comma in the argument list.  See
https://bugs.python.org/issue30399

This patch modifies tests that rely on this behaviour to not rely on it.
And the python34.py test is updated to include a test for this behaviour
with a .exp file.

In Python 3.7 "1 >> (big int)" is now allowed, it no longer raises an
OverflowError.  So use bytearray to test big-int conversion overflow.