net: add cpu idle utilization support for measurements
netperf: cpu utilization support

On base-foc and base-nova the cpu utilization is calculated based on the
idle cpu time. The rest utilization is reported as utilization for netperf.

Works solely for uni-processor setups.
lwip: enable SO_RCVBUF socket option

In issue #313 the SO_RCVBUF was intended to be enabled, however the current
lwip port looks for another define LWIP_SO_RCVBUF instead of LWIP_RCVBUF.

Fixes #716
netperf: use omni and rely on SO_RCVBUF available