scripts/mkits.sh: add support for adding DT overlay blobs to image

scripts/mkits.sh: add support for adding DT overlay blobs to image

Allow adding multiple device tree overlay blobs to an image and
generate configurations for each of them.
This is useful on boards with modern U-Boot which allow e.g. user-
configurable peripherals ("shields") in that way.
Note that currently, each generated configuration adds exactly one
overlay on top of the base image, ie. adding multiple overlays at the
same time is not yet supported.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>