build-linux.sh: Modify copy of linux- images to always pick latest

Multiple calls of "make bindeb-pkg" without calling do_clean() first
creates multiple versions of linux-* images with different sha appended
as version. This confuses the cp which now has multiple possible
candidates for linux-image-n1sdp.deb or linux-headers-n1sdp.deb files.
Therefore this patch makes sure that we find the latest image before
copying.

Signed-off-by: Sahil Kaushal <Sahil.Kaushal@arm.com>