board: cpu: Initial import of the Arduino Mega2560 (SWP)

## This pull request adds general support for the Arduino Mega 2560 board.
### This includes:

* **Atmel ATmega processors** in general (cpu/atmega_common)
* **Atmel ATmega2560** especially (cpu/atmega2560)
* **Arduino Mega 2560** (boards/arduino-mega2560)

### What is needed:
* avr-toolchain (tested on the one in the ubuntu repos and on avr-libc-1.8.0, binutils-2.23.2, gcc-4.9.0)
      Dependency on avr-libc right now is not optional.
* avrdude (for flashing the image to the board)
    
### What works:
* Taskswitching in general (tested with examples/ipc_pingpong)
* UART: (one USART is used as stdin/stdout, all four USARTs of the Arduino Mega 2560 should be supported)
* Timers (3 16 Bit Timers with 3 Channels each)
* Basic GPIO/LED (hardcoded)
 
### What is missing:
* LPM is not yet implemented
* GPIO interface is not implemented
* stdin is untested and should be reimplemented using interrupts

### Known issues:
* Several of the tests fail and are therefore blacklisted (bloom uses values larger that 2 Bytes, the others fail due to missing headers in avr-libc (time.h and sys/types.h).


*Note:* I've squashed the commits into 4 single commits, each dealing with one of the major code parts added (cpu/atmega_common, cpu/atmega2560, boards/arduino-mega2560 and tests). I hope that it's ok this way (otherwise I also could push the unsquashed commits which would be more chaotic thought).

 
this will not work - as you have seen. As the ATMega does not have a 'global' timer IRQ flag, you need to do some tricky parts here. The interrupts for each channel should only be enabled, if a timer channel is actually set. So when you call this function and not timer channel is in use, nothing should happen. If one channel is used at that moment, only that channel's interrupt should be enabled.

I would suggest to introduce a variable that keeps track of used timer channels and can then be used to determine on call of `timer_irq_enable` which channels actually have to be enabled, and the same of course for `timer_irq_disable`