Complete the definition of low-level radio driver API
Merge pull request #1211 from rousselk/complete-radio_driver_h

Completing low-level radio driver definition.
documentation: CMSIS cleanup

Put all CMSIS documentation into a dedicated subgroup of cpu.
documentation: moved network documentation to top level
documentation: added missing doxygen header to transceiver interface
documentation: fixed organization of 6LoWPAN doxygen
documentation: fixed sixlowpan doxygen filename