* 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

Byte code has a map from byte-code offset to source-code line number,
used to give better error messages.

Parser no longer prints an error, but instead returns an exception ID
and message.

Thanks to Dave Hylands for this.

* Add simple ADC driver, with support for ADC1 and all channels.
* Export MicroPython ADC object with read_channel function.
* Add stm32f4xx_adc.c and adc.c to Makefile.

vstr is initially intended to deal with arbitrary-length strings. By
providing a bit lower-level API calls, it will be also useful to deal
with arbitrary-length I/O buffers (the difference from strings is that
buffers are filled from "outside", via I/O).

Another issue, especially aggravated by I/O buffer use, is alloc size
vs actual size length. If allocated 1Mb for buffer, but actually
read 1 byte, we don't want to keep rest of 1Mb be locked by this I/O
result, but rather return it to heap ASAP ("shrink" buffer before passing
it to qstr_from_str_take()).

See pyb_usart_global_debug variable.

Also did some work on USB OTG, but nothing working yet.

Fixes Issue #132.

* Add Micro Python bindings to usart.c
* Export USART python object to namespace pyb

* Fix rtc_init to use LSI for RTC clock source when LSE is not detected.

Updated mconfigport.h to enable GC