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
cpu/arm_common: c++: initial support for c++ added _kill (weak) and weak attribute for _kill_r
boards/msba2: provided c++ feature (cpp)
Initial import of a DAC test application
shell: introduced l2 ping shell commands
l2_ping: add to auto_init
travis: check for doxygen warnings

This introduces a script that will check for doxygen warnings in newly
committed or edited files.
travis: added doxygen check script to travis config
Merge pull request #1495 from BytesGalore/EasyCppMixing_msba2_support_fix

cpu/arm_common C++ msba2 support fix
Merge pull request #1228 from OlegHahm/l2_ping

net: Link Layer Ping
Merge pull request #2023 from PeterKietzmann/add_test_periph_dac

test/periph_dac: adds a test for DACs
Merge pull request #2104 from OlegHahm/travis_doccheck

travis: check for doxygen warnings