It's really a UART because there is no external clock line (and hence no
synchronous ability, at least in the implementation of this module).
USART should be reserved for a module that has "S"ynchronous capabilities.

Also, UART is shorter and easier to type :)

The three classes I2C, SPI and USART now have a fairly uniform (Python)
API.  All are constructed, initialised and deinitialised in the same
way.  They can have most of their parameters set, using keyword arguments.
All have send and recv (although slightly different with I2C requiring an
address in master mode).  recv can do inplace receiving (ie store the
data in a previously-created bytearray).

It's just polling mode at the moment, but interrupt and DMA would be
nice to add.

Also some updates to compile with latest changes to core py.

It's Light Emitting Diode, so should be LED.

This is an attempt to clean up the Micro Python API on the pyboard.
Gpio functionality is now in the Pin object, which seems more natural.
Constants for MODE and PULL are now in pyb.Pin.  Names of some
classes have been adjusted to conform to CamelCase.  Other
miscellaneous changes and clean up here and there.

Reads ADC values into a bytearray (or similar) at a fixed rate.  Needs a
better name and improved API.  Also fix up DAC dma function (which also
needs a better name and API).

Simple way to find the address of an attached I2C device.

Also contains raw memory read/write functions, read8, read16, read32,
write8, write16, write32.  Can now do:

stm.write16(stm.GPIOA + stm.GPIO_BSRRL, 1 << 13)

This turns on the red LED.

With the new constant folding, the above constants for the GPIO address
are actually compiled to constants (and the addition done) at compile
time.  For viper code and inline assembler, this optimisation will make
a big difference.  In the inline assembler, using these constants would
not be possible without this constant folding.

Can now calibrate, set pulse width, angle and speed.

They correspond to io.FileIO in io module hierarchy (with small caveat
that io.FileIO is raw file and works with bytes, not strings).

This is to reduce ROM usage.  stream_p is used in file and socket types
only (at the moment), so seems a good idea to make the protocol
functions a pointer instead of the actual structure.

It saves 308 bytes of ROM in the stmhal/ port, 928 in unix/.

This adds readline(), readall(), etc. methods. Also other fixes to make
Python API compliant and "with" support.

Can now choose at boot up whether the USB device is CDC+MSC or CDC+HID.
Choice is made by an option in boot.py, with default being CDC+MSC.
HID+MSC is not currently supported, but should be easy to implement.

Boot up now has ability to change the reset mode: hold down USR switch
while booting and LEDs will count from 1 to 7 to indicate the boot mode.
Release USR when correct mode is selected.  Current modes are 1 (normal
boot), 2 (safe mode), 3 (reset FS mode).

Addresses issue #377.

Revamp usrsw to use new exti code.
Add user switch callback function.

This commit also introduces board directories and moves board
specific config into the appropriate board directory.

boards/stm32f4xx-af.csv was extracted from the STM32F4xx datasheet
and hand-tweaked.

make-pins.py takes boards/stm32f4xx-af.csv, boards/stm32f4xx-prefix.c,
and boards/BOARD-NAME/pins.csv as input and generates the file
build/pins_BOARD_NAME.c

The generated pin file for PYBOARD4 looks like this:
https://gist.github.com/dhylands/9063231

The generated pins file includes all of the supported alternate
functions, and includes upsupported alternate functions as comments.

See the commnet block at the top of stm/pin_map.c for details on
how to use the pin mapper.

I also went ahead and modified stm/gpio.c to use the pin mapper.

This is compatible with what search path was before sys.path refactor,
with addition of module library path ("0:/lib").

GPIO functions are crude, and need to be replaced with better ones!