The function mp_obj_new_str_of_type is a general str object constructor
used in many places in the code to create either a str or bytes object.
When creating a str it should first check if the string data already exists
as an interned qstr, and if so then return the qstr object.  This patch
makes the function have such behaviour, which helps to reduce heap usage by
reusing existing interned data where possible.

The old behaviour of mp_obj_new_str_of_type (which didn't check for
existing interned data) is made available through the function
mp_obj_new_str_copy, but should only be used in very special cases.

One consequence of this patch is that the following expression is now True:

    'abc' is ' abc '.split()[0]

This patch simplifies the str creation API to favour the common case of
creating a str object that is not forced to be interned.  To force
interning of a new str the new mp_obj_new_str_via_qstr function is added,
and should only be used if warranted.

Apart from simplifying the mp_obj_new_str function (and making it have the
same signature as mp_obj_new_bytes), this patch also reduces code size by a
bit (-16 bytes for bare-arm and roughly -40 bytes on the bare-metal archs).

At the original frequency of 90MHz there's no way to get a 48MHz USB clock.
These new setting mirror those of the STM32F429DISC board.

Rationale:

* Calling Python build tool scripts from makefiles should be done
  consistently using `python </path/to/script>`, instead of relying on the
  correct she-bang line in the script [1] and the executable bit on the
  script being set. This is more platform-independent.
* The name/path of the Python executable should always be used via the
  makefile variable `PYTHON` set in `py/mkenv.mk`. This way it can be
  easily overwritten by the user with `make PYTHON=/path/to/my/python`.
* The Python executable name should be part of the value of the makefile
  variable, which stands for the build tool command (e.g. `MAKE_FROZEN` and
  `MPY_TOOL`), not part of the command line where it is used. If a Python
  tool is substituted by another (non-python) program, no change to the
  Makefiles is necessary, except in `py/mkenv.mk`.
* This also solves #3369 and #1616.

[1] There are systems, where even the assumption that `/usr/bin/env` always
exists, doesn't hold true, for example on Android (where otherwise the unix
port compiles perfectly well).

All the asm macro names that convert a particular architecture to a generic
interface now follow the convention whereby the "destination" (usually a
register) is specified first.

Recent vendor SDKs ship libs with code in .text section, which previously
was going into .irom0.text. Adjust the linker script to route these
sections back to iROM (follows upstream change).

The SHA1 hashing functionality is provided via the "axtls" library's
implementation, and hence is unavailable when the "axtls" library is not being
used.  This change provides the same SHA1 hashing functionality when using the
"mbedtls" library by using its implementation instead.

As vars contains array of base types and its length, name them as such,
avoid generic "items" and "len" names.

Doesn't list specific API calls yet, the purpose is to let user know that
the module exists.

Macros to convert big-endian values to host byte order and vice-versa.
These were defined in adhoc way for some ports (e.g. esp8266), allow
reuse, provide default implementations, while allow ports to override.

Plus, additional descriptions/formatting.

Commented out by default.

In the vendor SDK 2.1.0, some of the functions which previously didn't
have prototypes, finally acquired them. Change prototypes on our side
to match those in vendor headers, to avoid warnings-as-errors.

If SSL_EAGAIN is returned (which is a feature of MicroPython's axTLS fork),
return EAGAIN.

Original axTLS returns SSL_OK both when there's no data to return to user
yet and when the underlying stream returns EAGAIN. That's not distinctive
enough, for example, original module code works well for blocking stream,
but will infinite-loop for non-blocking socket with EAGAIN. But if we fix
non-blocking case, blocking calls to .read() will return few None's initially
(while axTLS progresses thru handshake).

Using SSL_EAGAIN allows to fix non-blocking case without regressing the
blocking one.

Note that this only handles case of non-blocking reads of application data.
Initial handshake and writes still don't support non-blocking mode and must
be done in the blocking way.

The technique of using alloca is how dotted import names are composed in
mp_import_from and mp_builtin___import__, so use the same technique in the
compiler.  This puts less pressure on the heap (only the stack is used if
the qstr already exists, and if it doesn't exist then the standard qstr
block memory is used for the new qstr rather than a separate chunk of the
heap) and reduces overall code size.

This reverts commit 3289b9b7.
The commit broke building on MINGW because the filename became
micropython.exe.exe.  A proper solution to support more Windows build
environments requires more thought and testing.

And describe an alternative of using uselect.poll().

Fixes a few C warnings.  No functional changes.

Per the comment found here
https://github.com/micropython/micropython-esp32/issues/209#issuecomment-339855157,
this patch adds finaliser code to prevent memory leaks from ussl objects,
which is especially useful when memory for a ussl context is allocated
outside the uPy heap.  This patch is in-line with the finaliser code found
in many modsocket implementations for various ports.

This feature is configured via MICROPY_PY_USSL_FINALISER and is disabled by
default because there may be issues using it when the ussl state *is*
allocated on the uPy heap, rather than externally.

A step towards implementing non-blocking stream support for SSL.

With inplace methods now disabled by default, it makes sense to enable
reverse methods, as they allow for more useful features, e.g. allow
for datetime module to implement both 2 * HOUR and HOUR * 2 (where
HOUR is e.g. timedelta object).

This allows to configure support for inplace special methods separately,
similar to "normal" and reverse special methods. This is useful, because
inplace methods are "the most optional" ones, for example, if inplace
methods aren't defined, the operation will be executed using normal
methods instead.

As a caveat, __iadd__ and __isub__ are implemented even if
MICROPY_PY_ALL_INPLACE_SPECIAL_METHODS isn't defined. This is similar
to the state of affairs before binary operations refactor, and allows
to run existing tests even if MICROPY_PY_ALL_INPLACE_SPECIAL_METHODS
isn't defined.