Add a way to determine whether a trickle timer is running
Only schedule a trickle interval doubling for running timers
Cleaned up various fragments and fixed a compilation error that occured when switching metric container.

Fixed a wrapping problem in the ETX EWMA calculation.

Corrected the multiplier of the link metric, and simplified the configuration
so that the user does not need to specify the multiplier.
Added SPI driver for CC2538

This uses the core/dev/spi.h header and implements the spi_init()
function and the various macros for SPI operation. ssi.h contains all of
the register locations and information.

This implementation is not very versatile, mostly because I don't how to
make it flexible in the contiki system. It supports pin muxing for the
four spi pins, but other than that picks sensible defaults.

The SPI macros (like SPI_READ()) are defined in
cpu/cc2538/spi-arch.h. In order to use the SPI driver, add the following
includes to your project:

    #include "spi-arch.h
    #include "dev/spi.h"
Merge pull request #352 from lab11/cc2538-spi-driver

Added SPI driver for CC2538
Use TRICKLE_TIMER_IS_STOPPED in trickle_timer_stop()
Merge pull request #359 from g-oikonomou/trickle-timer-stop

Add a method to stop running trickle timers
Set peripheral mode for the correct UART TX/RX pins
Saved another byte ;-)
Merge pull request #368 from oliverschmidt/master

Saved another byte ;-)
Adjusted Makefile to the final 'atarixl' cc65 target.

- Assign source files to the splitted shadow RAM areas.
- Switched to dynamic Ethernet driver loading.
Merge pull request #369 from oliverschmidt/master

Adjusted Makefile to the final 'atarixl' cc65 target.
Removed support for 'atari' now that 'atarixl' is part of the cc65 master.
Merge pull request #370 from oliverschmidt/master

Removed support for 'atari' now that 'atarixl' is part of the cc65 master.
Some beautification for the Contiki online configurator.
Merge pull request #371 from oliverschmidt/master

Some beautification for the Contiki online configurator.
Adjustments for the switch from 'atari' to 'atarixl'.
Merge pull request #372 from oliverschmidt/master

Adjustments for the switch from 'atari' to 'atarixl'.
Added missing 'atarixl' Makefile defines.
Merge pull request #373 from oliverschmidt/master

Added missing 'atarixl' Makefile defines.