samples: net: perf: Add sample for throughput measurement

Add a simple UDP server application that will receive UDP data
and print various statistics like number of packets received and
dropped, amount of bytes received, and how many packets / sec
we were able to receive.

Currently the sample only provides a config file for FRDM-K64F board.
Note that this sample should not be run in QEMU as the performance
is very bad there because of SLIP link between Linux host and QEMU.

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>