Update LED configuration

Print an error if an invalid BOARD is specified

Add EXTI support

Fix floating point printing of negative numbers in the range (0, -1.0)

Also change int -> machine_int_t where appropriate.

With this option selected, only HID on its own works, not VCP+HID.

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

Stack layout is different when -g used, so need to handle the pendsv
jump differently.  Addresses Issue #315.

Add Configurable LFN support to FatFS

Some functionality is still missing (eg and, or, bit shift), and some
things are buggy (eg subtract).

strtonum clashes with BSD function of same name, and our version is
different so warrants a unique name.  Addresses Issue #305.

parse: Refactor parse node encoding to support full range of small ints.

Based on suggestion by @dpgeorge at
https://github.com/micropython/micropython/pull/313

Specifically, VM's small ints are 31 bit, while parser's only 28. There's already
MP_OBJ_FITS_SMALL_INT(), so, for clarity, rename MP_FIT_SMALL_INT() to
MP_PARSE_FITS_SMALL_INT().

TODO: De-duplicate DECODE_UINT, etc. definitions.

Assuming we have truncating (floor) division, way to do ceiling division
by N is to use formula (x + (N-1)) / N. Specifically, 63 bits, if stored
7 bits per byte, require exactly 9 bytes. 64 bits overflow that and require
10 bytes.

Support varlen encoding for arbitrary int and uint numbers

Add pin mapping code.

http://docs.python.org/3.3/library/functions.html#__import__ :
"When the name variable is of the form package.module, normally, the
top-level package (the name up till the first dot) is returned, not
the module named by name. However, when a non-empty fromlist argument
is given, the module named by name is returned."

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.

To follow up pull request #309.