Hi guys !

I would like to know how we can fully encrypted the system.

I've checked some stuffs and i don't understand everything.
Does u-boot can work with dm-crypt, luks ?
I've check in /boot the config file for the kernel and dm-crypt is marked as "m" (module)

but firstly : is it possible to do ?

It should be possible to do, although I have not tried it, with the correct modules and scripts set up in the initrd. None of the bootable SD card images actually support installing in this manner though so you would be stuck doing it the hard way which would be quite a pain to do manually for most people.

/boot would remain unencrypted though because otherwise you would not be able to load the kernel and initrd. Debian pretty much does the same when you install it with the LUKS encrypted option.