New platform: Mulle

This is a pull request for adding the Mulle ARM based platform to the Contiki repository. It mostly adds new files for the platform and cpu parts used by the Mulle.

Currently our users are Luleå University of Technology
and several industry partners wishing to develop wireless
functionality for their sensor applications.

About the platform:

The Mulle platform is based around a Freescale Kinetis K60 processor
(ARM Cortex M4) and the Atmel RF212 radio (rf230 driver). The board
designed for prototyping IoT applications, it is quite small (21 mm x
34 mm) and contains everything that is needed to build a wireless
sensor network. The primary focus of the board is to provide a
flexible prototyping platform for battery powered IoT applications.
Since we are focusing on battery powered applications the hardware
needs to be extremely power efficient and the software needs to
recognize this in order to optimize radio usage etc. Contiki provides
a good framework for achieving these goals.

Previously Eistec has been working with Contiki on the M16c MCU, which
is quite a lot less versatile and powerful than the ARM Cortex M4 core
used in the current revision. Development of the new version is backed
both by our in-house developers and researchers at Luleå University of
Technology.