@por I did it with dd if=/media/flash/update/uImage-initramfs of=/dev/mmcblk0 bs=512 seek=12288
@pkbo I think M92 sees the partitions on sd card, even you don't see them when opening sd card icon. You can easily check through serial connection. I'm leaving the boox to bootup, maybe it takes some hours
I think I messed something up with the symbolic links or my sd card is really really sloow (I don't know what class it is)