make: make clean usable together with `-j`
Merge pull request #1201 from Kijewski/exclusive-make-clean

make: make clean usable together with `-j`
net: move ping to sys/net

* moved the former ping module from `sys` to `sys/net/link_layer` and renamed to l2_ping
* use defaulttransceiver instead of cc110x
* some refactoring
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: enabed 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