cleanup-image: smart image shrink using fstrim

Linux loop driver support TRIM operation for some time (from 3.1
kernel), which is mapped to hole punch operation in underlying
filesystem (which must support it - ext* and xfs have it).
This is much faster and less space consuming than previous cp --sparse
method.

cleanup_image

@@ -23,11 +23,7 @@ export INSTALLDIR=`pwd`/mnt/
 $SCRIPTSDIR/09_cleanup.sh
 
 echo "--> Compacting image file..."
-dd if=/dev/zero of=mnt/fillme bs=1M > /dev/null 2>&1 || true
+fstrim mnt/
-sync
-rm mnt/fillme
 umount mnt
-cp --sparse=always $IMG $IMG.new
-mv  $IMG.new $IMG
 ls -als $IMG