As per issue #257, servo is better on TIM5 because TIM2 is connected to
more GPIO.

All board config macros now begin with MICROPY_HW_.

Renamed PYBv10 to PYBV10, since macros should be all uppercase.

Made SDCARD_DETECT configurable in mpconfigport.h, so that the SD
detect pin can be easily configured.

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).

Adds readline_init() to clear readline history on soft reset.  Addresses
issue #387.

Comes with some refactoring of code and renaming of files.  All modules
are now named mod*.[ch].

Added support for the ADC channels and mappings to make_pins.py

I'm not sure if the hal properly deals with the channel 16/18 differences
between the 40x and 42x. It seems to deal with it partially. This particular
aspect will need testing on a 42x or 43x.

This gives a functioning, independent MSC device.

Just low-level read/write support.  No filesystem yet.

New USB HAL is quite a bit improved over previous one.  Now has better
callbacks and flow control.

REPL over USB CDC now works as before, except for soft-reset (since USB
driver uses malloc...).

Enumerates CDC device over USB, but doesn't transmit/receive yet.

This compiles and links, but hasn't been tested on a board
yet and even if it was run, it doesn't currently do anything.

mp_module_obj_t can now be put in ROM.

Configuration of float type is now similar to longint: can now choose
none, float or double as the implementation.

math module has basic math functions.  For STM port, these are not yet
implemented (they are just stub functions).

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.

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.

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

Raw REPL mode is activated by CTRL-A, and allows you to more easily
communicate with the pyboard over the serial channel.  In particular,
automated communication using, eg, pyserial on the PC side.