UDP test

This test is largely based on the <code>gnrc_networking</code> example but uses a modified udp command: Instead of providing data in a string format a number of bytes is given. A packet of that size is then send, its content consisting of an incrementing repeating byte. The server is also modified as it outputs the number of currently received packets when a new packet is received instead of the content of the new packet. This counter can be reset using udp reset