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
shell: introduced l2 ping shell commands
l2_ping: add to auto_init
Merge pull request #1228 from OlegHahm/l2_ping

net: Link Layer Ping