The "CHECKSUM" command(key1) calculates a pair of check values for the master IC.
These values are used for identify this specified IC.

Two values were be shown on the LCD display. The upper one is summed from the
one's complement of each byte. The lower one is summed directly from each byte.
The first one has the advantage that for a blank chip the value will always be
"0000" no matter what the size of this chip is.

One long and one short beep will be issued to warn the use about a checksum change,
and a leading "*" character will shown together with the new value.