Thanks for the pointer.
When I built the rpi3 image, I used the existing rpi2 image we had,
including the config.txt that was used.

>From that wiki page, it seems that we're really under what both the
pi2/p3 can support, including default values.
Maybe we should at least bump this to "default" values (so not
overclocking).
So arm_freq set to 900 for pi2 and 1200 for pi3

Opinions ?