- ATmega based boards: Added parameter for W5100 to board_common.h
- Arduino Due based boards: Use board.h instead of providing w5100_params.h

The `atmega_set_prescaler` was using a "sensible" default
but it's better to define it at the board level to make
it clear.

This adds a LED_PANIC macro which defines which LED,
or combination of LEDs should notify a panic error.
This is currently used to signal BADISR_vect errors.

    - auto set XTIMER_SHIFT relative to XTIMER_HZ
    - refine setting of XTIMER_WIDTH

This was previously configured to PC5 which is shared with the i2c
SCL line. PD7 is selected since it's the only pin which does not
have an additional function.

Refactors boards/arduino-common to boards/arduino-atmega-common.
Extends boards/arduino-atmega-common to be generic to all atmega based
arduino boards, not just atmega328p.
Removes flags that should be located in cpu framework

It wasn't working with pin1/PC6/PINT14
Use pin28/PC5/PINT13 instead

Uno and Duemilanove(atmega328p version) are nearly the same boards.
The only difference is that the Duemilanove use an FTDI usb chip,
while the Uno use an Atmel which acts as USB/Serial converter.
All of the code needed to support these boards is in arduino-common.

Fixes #5745
For AVR based boards, three defines must be defined AVR_CONTEXT_SWAP_INIT,
AVR_CONTEXT_SWAP_INTERRUPT_VECT, and AVR_CONTEXT_SWAP_TRIGGER.
These defines are used to trigger a software interrupt used for context
switching.

When AVR_CONTEXT_SWAP_INTERRUPT_VECT is handled, the scheduler is run
and a context swap will happen if necessary, with the resulting thread
starting following the reti instruction. This results in threads running
at normal priority instead of at interrupt priority.

Atmega devices do provide a pure software interrupt. The method used
here for waspmote-pro and arduino-mega2560 is to use pin change
interrupts, set the pin to act as an output, and toggle the value to
simulate a software interrupt. The main limitation here is that a
physical pin is now occupied and must be defined for each board
supported by RIOT. On the plus side, it provides an easy method for
detecting context swaps with an oscilloscope.

added a pin mapping file that maps MCU GPIO pins to Ardunio
specific pin numbers

* also added a trailing underscore to header guards for consistency

Commit for PR 2623, repairing header file include guards.
This PR is intended to fix the include guards in files under RIOT/boards

SQUASH ME: fix underscore removal overdos

SQUASH ME: consistent macro naming

SQUASH ME: missed that one

SQUASH ME: fixed overdo

SQUASH ME: consistency

for
- sam3x8e
- stm32f0
- stm32f4
- sam3x8e
- nrf51822

- fixed comment style and bracketing
- disabled newlib nano, added mac support
- fixed OSX compatibility in Makefile.include