graph-log: Add more formatting tweaks, include drift ppm

Bunch of formatting tweaks and adding of the drift ppm to
the chart.

Signed-off-by: John Stultz <john.stultz@linaro.org>
ntp-convergence: First pass at ntp convergence test

Simple test that injects a fixed offset and logs as ntp
corrects the offset.

Still likely a number of improvments to be made, but
this seems to work ok.

Signed-off-by: John Stultz <john.stultz@linaro.org>
drift-log.py: Fixup loop exit

Avoid sleeping for the interval time after the last query is done.

Signed-off-by: John Stultz <john.stultz@linaro.org>