of an rtimer for the periodic radio duty cycling. The benefit of this
implementation is that it leaves an rtimer free for user tasks. The
drawback is a slightly higher idle power consumption. This new version
is based on the current rtimer-based X-MAC.
the packetbuf from packet attributes. Two framer modules are included,
one that creates 802.15.4 frame headers, and one that creates a
minimal "nullmac"-style header that only contains the link-local
addresses.