Extend tools/readserial.py to support serial port timing calibration.

Extend tools/readserial.py to support serial port timing calibration.

Some serial ports have slightly different timing.  These timing
variations result in less accurate boot time reporting.  So, add a
calibration mechanism to the tool so that one can determine how much
time a specific machine's serial port uses.

Also, extend the main part of the tool to allow the user to specify
an exact timing parameter.