Thank you guys for your answers.
@Tapani: I tried with new SD cards (never been used before), it still does not help. I also tried with various images: Android, Ubuntu, Yocto, etc., still no result. I also checked and my SD cards work perfectly fine with other boards (Rpi for example).
@ClayMontgomery: thanks for the idea.I also tried that (with Ubuntu and Yocto images), and the problem still remains
At that point, after several weeks of trying for nothing, I still have no clue about what is wrong with those boards, and I am simply going to send them back and to switch to boards from an other manufacturer.