Removes entries from the final update.cfg file for which no image was generated.

Removes entries from the final update.cfg file for which no image was generated.

Until now, the update.cfg file has contained entries even when the
related image file was not generated due to lack of differences. For
example if there were no differences between Tizen versions for boot.img
then the image as not placed in delta.tar file, but in update.cfg the
entry associated with it remained.

This patch removes entries for which there is no corresponding image. In
case the resulting update.cfg (e.g. an update between the same
versions), the mk_delta.sh script will end with an error.

Change-Id: I61278a8f1efb69bd908b1dd1aae5ebf714963f6e