Reads characters from the serial port. The string is allowed to contain any character, including 0. Note that the returned string may not end in a newline if this character is not present in the buffer.
@status current Introduced in 2.3.8
num (optional): maximum number of bytes to read.
If non-zero, serialRead will read up to num characters from the buffer.
If 0 or left out, serialRead will read up to and including the first newline character or the end of the buffer.
string Empty if no new characters were available.