Install DTB as part of images install command

When installing DTB to the images directory, do this as part of the image
install commands rather than the target install commands.

Signed-off-by: Reuben Dowle <reuben.dowle@4rf.com.az>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>