rpl: removing unnecessary recv_mutex

In the main function of RPL (rpl_process thread)
a mutex is used after receiving a message via msg_recv.
However, this mutex is never used in another thread and thus is rendered
obsolete.
Merge pull request #2147 from cgundogan/rpl_unnecessary_mutex

rpl: removing unnecessary recv_mutex
add driver for tmp006 thermopile sensor
add test app for tmp006 sensor