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.

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!

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.

Each built-in exception is now a type, with base type BaseException.
C exceptions are created by passing a pointer to the exception type to
make an instance of.  When raising an exception from the VM, an
instance is created automatically if an exception type is raised (as
opposed to an exception instance).

Exception matching (RT_BINARY_OP_EXCEPTION_MATCH) is now proper.

Handling of parse error changed to match new exceptions.

mp_const_type renamed to mp_type_type for consistency.

Addresses issues #272 and #273.

If SD card is present on (soft) reset then it's mounted on 1:/ and files
can be openned using, eg, open('1:/test.txt', 'r'), or 'w' for writing.

This also fixes up the IRQ for the PYBOARD4 USERSW
although I was unable to test that functionality.

Using PendSV interrupt at lowest priority, code can now raise an
exception during an interrupt by calling pendsv_nlr_jump.  The exception
will be raised when all interrupts are finished.  This is used to trap
ctrl-C from the USB VCP to break out of running Python code.

* Add SD card test to fix warning

* Issue #232

To partly address Issue #220.

This reverts commit a215b09c.

* Move gc_collect from main to py/gc.c
* Define GC's memory boundaries in linker script
* Issue #220

* Move RTC code from main to a separate module.

* Add MICROPY_HW_HAS_LCD
* Add MICROPY_HW_HAS_WLAN
* Use feature test macros in main

* Add some generic feature macros to mpconfigport
* Move pin and port definitions from usrsw.c to mpconfigport

f2d and d2f functions from libgcc does not work correctly, most likely
due to the ABI being incorrect.  libgcc disabled for now.

Can now have null bytes in strings.  Can define ROM qstrs per port using
qstrdefsport.h