Remove an excess cp of disk images.

save_image is used to copy kernel and ramdisks out of the image, which
we will sometimes want to keep the source, and sometimes not. However
for the main image itself, the temp copy is never kept, so use mv
  rather than cp and avoid the excess IO.

Change-Id: I5a9f0d69ffee3e6b872a8927537ac17f02f5aa4d
Merge "Remove an excess cp of disk images."