Really good point, my example can be misleading. I didn't wanted to add confusion by fully explaining my goal.

The goal is to run extlinux --install ${IMAGE_ROOTFS}/boot/syslinux, as I've understood, this command install the sylinux required file but also does write some bytes at the start of the partition, for the MBR. In debbos, I could do that using the following code and now I'm trying to reproduce that in the Yocto build system: