Checks for the presence of a UDP packet, and reports the size.
parsePacket() must be called before reading the buffer with
It's usually more efficient to use
receivePacket() instead of
// PROTOTYPE int parsePacket(system_tick_t timeout = 0); // SYNTAX size = Udp.parsePacket();
timeout: The timeout to wait for data in milliseconds, or 0 to not block in Device OS 2.0.0 and later. Prior to 2.0.0 this function did not block
int: the size of a received UDP packet