nitrogen6x: 6x_bootscript: load dtb using

Signed-off-by: Eric Nelson <eric.nelson@boundarydevices.com>
boundary bootscripts: Use  for loading DTB in single-partition scripts

Signed-off-by: Eric Nelson <eric.nelson@boundarydevices.com>
nitrogen6x bootscript: use bootpart for uImage

Signed-off-by: Eric Nelson <eric.nelson@boundarydevices.com>