The `udp` command is a valuable debugging tool that is also useful
outside of the gnrc_networking example.
To enable easy sending of udp messages in other applications during
development, move the `udp` command to the shell module and introduce
the `gnrc_udp_cmd` pseudo-module to enable it.
Since it is likely that the main thread will send netapi IPC calls that expects a reply. These replies may come faster than the thread can handle them, causing the layers below to stuck.
