Extracted ld section layout to dedicated file
More fixes to dependency tracking code that still sucks
Use MBED board only for lpc_17xx mcu
stm32f4 code is now functional, more improvements

 * stm32f4 code is functional, added default board config for
   stm32f4discovery, app_empty and app_blink are working;
 * refactored Timer code, wait() is now implemented in the trait;
 * updated gdbinit with helper functions for stm32f4 debugging;
 * added basic support for code configuration via Rakefile's features
   and platform-specific features.

Notes on stm32f4:

The only tested configuration is 168MHz, others should possibly work as
well.

All the gpio module code is implemented in pin.rs, as stm32f4 implements
pin mode selection in totally different way.

There's no pin map yet, the thing is huge.

Timer support is *very* basic, only Timer2 for now just to make
app_blink work. Why this thing has so many timers? Sometimes I feel like
working with cortex-a.

LLVM's new __morestack is broken on stm32f4 as well.
Removed old demo code