After a few days of work, here's what we currently have:

Done the last weeks:

• Installed Arch Linux instead of Raspbian Wheezy : the boot time goes from 30 seconds to ... 10 seconds, waw! (In fact, the reason for this speed boost is not really ArchLinux vs. Raspbian, but the use of the new systemd boot management system; it is now available by default on Raspbian Jessie)

• the Digitast buttons (for Preset +, Preset -) now work

• After a few unsucessful attempts, the 7-segment display now works!