mmem: Prevent duplicate init of mmem

This could seriously corrupt data if mmem_init was called again after
someone called mmem_alloc.
Enabled 2nd button on platform econotag
Fix time accounting on msp430 Series 1 and Series 2 MCU based platforms.

The problem with the current version of the code was that the condition at the end of the do...while loop at Timer A1 interrupt:
 while((TACCR1 - TAR) > INTERVAL);
evaluates to false whenever TACCR1 == TAR.
Not incrementing TACCR1 in this case leads to Timer A1 interrupt not being called for 2 seconds, until TAR counter reaches TACCR1 again after an overflow.

The patch avoids this problem by changing the condition of the loop, and using CLOCK_LT macro to compare between time values.

The patch also attempts to fix another problem: a read of TAR register while it is being updated may return a lower value than the actual contents. To avoid that, the "read twice and compare results" idiom should be used. As the TAR register is updated by the actual hardware, it is of no importance whether it is read with interrupts disabled or enabled; the problem can occur in both contexts.
Add PLATFORM_HAS_RADIO in Sky Platform

In order to use the er-server-example Radio resource it is required that the platform defines that it has a radio. This line might be required in other platforms.
[avr] added missing mcu parameter to ASFLAGS in order to enable assembler compilation
[cooja] skins/TrafficVisualizerSkin: Added @Override annotations and added some final modifiers
[cooja] skins/TrafficVisualizerSkin: Indicate sent but unreceived messages by a red double circle around mote
[cooja] skins/TrafficVisualizerSkin: Use LinkedList instead of ArrayList to increase add and remove performance
[cooja] skins/TrafficVisualizerSkin: Removed unnecessary 'historyCopy'
[cooja] skins/TrafficVisualizerSkin: Reduce array copy and iteration overhead
[cooja] skins/TrafficVisualizerSkin: Age handling moved to RadioConnectionArrow class which also got some lines of function documentation
[cooja] skins/TrafficVisualizerSkin: Use alpha value to fade out arrow color an potentially allow to configure colors
[cooja] plugins/sinks/TrafficVisualizerSkin: Synchronized to avoid ConcurrentModificationExcpetions
[cooja] plugins/Visualizer: Simple mote tooltip

Shows a motes type identifier and filename of firmware/sourcecode running on it
[cooja] Added @Override annotations for relevant files
[cooja] Added MemoryLayout class to represent basic information about
motes memory such as word size or endianess
[cooja] Added base class `MemoryBuffer` for reading and writing both simple and complex data structures
to and from memory with support for padding.
[cooja] memory: Introduce minimal memory access interface 'MemoryInterface'

A mote should provide access to its memory using this interface.
All high-level access methods are wrapped around this minimal interface.

Contains dummy-implementations of functions for AvrMoteMemory and MspMoteMemory
[cooja] memory: AvrMoteMemory and MspMoteMemory dummy implementation of MemoryInterface
[cooja] memory: Added address and symbol based memory access classes