Reads UDP data from the specified buffer. If no arguments are given, it will return the next character in the buffer.
This function can only be successfully called after WiFiUDP.parsePacket().
WiFiUDP.read(); WiFiUDP.read(buffer, len);
buffer: buffer to hold incoming packets (char*)
len: maximum size of the buffer (int)
b: the characters in the buffer (char) size: the size of the buffer -1: if no buffer is available