Adapt image generation scripts to new approach

There is no more boot image, instead we flash the MBR and the boot
loader directly.

The kernel partition has been renamed to system partition, since we
will be moving the rootfs into it at some point.
assembly_images.sh: Delete output image in case of errors

This ensures that if the output image is incomplete, we avoid the risk
that the user overlooks the error message and starts using the image.