net: l2_ping uses radio_packet_t instead of packet_info_t
net: implement L2 ping and pong handler

* also defines L2 payload types
net: renamed handler thread for l2_ping
net: streamlined functions and data types for l2 ping
net: renamed internal variable in l2 ping
net: enabled l2 ping in auto_init

* fixed module dependencies
net: add count parameter to l2_ping
net: created define for default l2 ping interval

* also updated copyriot and some doxygen stuff
net: cleaner interface for l2 ping

* made ping sending mutual exclusive
* do not print but provide statistic interface
timex: added uint64_t->timex_t conversion function
net: complete l2 ping stat calculation
documentation: added l2 ping documentation
net: make l2 ping interval configurable
net: introduced sequence number to l2 ping
net: introduce l2 probing packets and statistics
net: add payload parameters to l2 ping functions

fixed counting and sent flag
shell: introduced l2 ping shell command
net: added debugging for l2_ping
shell: add handlers for link layer probing