mp_kbd_exception is now considered the standard variable name to hold the
singleton KeyboardInterrupt exception.

This patch also moves the creation of this object from pyb_usb_init() to
main().

It can be overridden by a board if needed.

This is a pure refactoring (and simplification) of code so that stmhal
uses the software SPI class provided in extmod, for the machine.SPI
implementation.

So long as a port defines relevant mp_hal_pin_xxx functions (and delay) it
can make use of this software SPI class without the need for additional
code.

To make moduselect be usable by any port.

This allows one to construct an I2C object using ids that are specific
to the stmhal port, eg machine.I2C('X').  Right now the implementation
of I2C uses software I2C but the idea is to just change the C-level I2C
protocol functions to hardware implementations later on.

Implementations of persistent-code reader are provided for POSIX systems
and systems using FatFS.  Macros to use these are MICROPY_READER_POSIX and
MICROPY_READER_FATFS respectively.  If an alternative implementation is
needed then a port can define the function mp_reader_new_file.

Also adds "machine" to the list of modules that the parser can search for
constants.

This includes making sure that utime_mphal's sleep_ms() and sleep_us()
don't sleep on negative arguments.

This type was used only for the typedef of mp_obj_t, which is now defined
by the object representation.  So we can now remove this unused typedef,
to simplify the mpconfigport.h file.

These are *defined* per-port, but why redeclare them again and again.

This patch adds support to fsusermount for multiple block devices
(instead of just one).  The maximum allowed is fixed at compile time by
the size of the fs_user_mount array accessed via MP_STATE_PORT, which
in turn is set by MICROPY_FATFS_VOLUMES.

With this patch, stmhal (which is still tightly coupled to fsusermount)
is also modified to support mounting multiple devices   And the flash and
SD card are now just two block devices that are mounted at start up if
they exist (and they have special native code to make them more
efficient).

For these 3 bitwise operations there are now fast functions for
positive-only arguments, and general functions for arbitrary sign
arguments (the fast functions are the existing implementation).

By default the fast functions are not used (to save space) and instead
the general functions are used for all operations.

Enable MICROPY_OPT_MPZ_BITWISE to use the fast functions for positive
arguments.

Functions added are:
- randint
- randrange
- choice
- random
- uniform

They are enabled with configuration variable
MICROPY_PY_URANDOM_EXTRA_FUNCS, which is disabled by default.  It is
enabled for unix coverage build and stmhal.

Seedable and reproducible pseudo-random number generator. Implemented
functions are getrandbits(n) (n <= 32) and seed().

The algorithm used is Yasmarang by Ilya Levin:
http://www.literatecode.com/yasmarang

They are no longer needed because stm constants can now be 32 bits wide.

This uses the newly factored machine_mem functions.

To let unix port implement "machine" functionality on Python level, and
keep consistent naming in other ports (baremetal ports will use magic
module "symlinking" to still load it on "import machine").

Fixes #1701.

This is refactoring to enable support for the two USB PHYs available on
some STM32F4 processors to be used at the same time. The F405/7 & F429
have two USB PHYs, others such as the F411 only have one PHY.

This has been tested separately on a pyb10 (USB_FS PHY) and F429DISC
(USB_HS PHY) to be able to invoke a REPL/USB.  I have modified a PYBV10
to support two PHYs.

The long term objective is to support a 2nd USB PHY to be brought up as a
USB HOST, and possibly a single USB PHY to be OTG.

Port specific settings defined in mpconfigport.

py/mphal.h contains declarations for generic mp_hal_XXX functions, such
as stdio and delay/ticks, which ports should provide definitions for.  A
port will also provide mphalport.h with further HAL declarations.