pybootchartgui: show elapsed time for each task

Currently, finding the elapsed time of each task in buildtimes.svg
is a manual effort of checking the top axis and finding and subtracting
the end and start time of the task.

This change adds the elapsed time for each task, so that
manual effort of comparing start/end time is avoided.

Signed-off-by: Mauro Queiros <Mauro.Queiros@criticaltechworks.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>