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
'free_packet' in csma.c conflicted with 'free_packet' in maca.c
llsec: Added a layer in between NETSTACK_MAC and NETSTACK_NETWORK for implementing link layer security
llsec: Configuration option for defining a constant LoWPAN-wide security level
Sky: Calling NETSTACK_LLSEC.bootstrap from contiki-sky-main.c
Sky: DEBUG on/off switch
packetbuf: Made packetbuf_hdrlen applicable to inbound packets, too
framer-802154: Style fixes
framer-802154: Introduced mnemonic variable
framer-802154: Support for creating and parsing frames with security headers
framer-802154: Support for setting and accessing the frame type
attribute
framer-802154: Sequence number fix
framer-802154: Renamed variable for the sake of consistency
framer-802154: Simplification of frame802154_create
framer-802154: Conditional compilation of security-related code
framer-802154: Support for explicit keys
llsec: Calling NETSTACK_LLSEC.on_frame_created()
llsec: Allow for sharing endianness functions