Ok so I have done many test and the result is that I finally giveup on the old method (the one that need too dd on the sdcard) and I pull the last modification you have done in db-boot-tools (really useful when everything is setup).
I finally understand that the problem came from the formating (I haven't check if with the old method it was cut in 10parts) but the following command line in the tuto need to be changed :
1/ "abootimg -u boot-XXX.img -c "cmdline=root=/dev/mmcblk1
p9p10 rw rootwait console=ttyMSM0,115200n8"
With the new feature in db-boot-tools you also need to review the tutorial on "sudo ./mksdcard.sh ..." because the folder changed and you don't have the choice to provide the rootfs and the boot now (I don't know if it could matter or not)
Also I had to modify "db-boot-tools/dragonboard410c/linux/partitions.txt" line rootfs "rootfs,1024000,,,fastboot,rootfs.img" I fixed the flashed size at 1Go because when flashing the card it gave the error "error: file_write: write: No space left on device"
After what I successfully booted and logged on the SD card.