I finally managed to upgrade the RaspberryPi image on YunoHost 2.2 \o/
You can download it as usual: https://build.yunohost.org/yunohost4rpi2.img.7z
We had an issue at RaspberryPi booting + an incompatibility between the RPi B/B+ and YunoHost 2.2. All fixed now.
Edit: If you already have a Raspberry Pi B/B+ running and you want to upgrade it to YunoHost 2.2, please follow the instructions of this topic