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
[avr] added missing mcu parameter to ASFLAGS in order to enable assembler compilation
[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
[cooja] memory: SectionMoteMemory dummy implementation of MemoryInterface
[cooja] getMemory() now returns MemoryInterface

Including code adaptions based on new memory methods to keep code compilable
[cooja] memory: Removed MoteMemory class including all references
[cooja] memory: Removed AddressMemory class including all references
[cooja] memory: Moved SectionMoteMemory to package memory
[cooja] Implementation of former AvrMoteMemory dummy functions
[cooja] Implementation of former MspMoteMemory dummy functions
[cooja] ContikiMoteType: Non-functional indention and code style updates
[cooja] ContikiMoteType: Some renames and function reordering
[cooja] memory: Implementation of simple array-backed memory class 'ArrayMamory'
[cooja] memory: SectionMoteMemory implementation of MemoryInterface functions
as forwarding to encapsulated memory sections

Requires to make use of Symbol class for cooja motes, too