disk-image-create: fix test when no argument is given.

Avoid a warning when multiple arguments are passed to disk-image-create
like:

bin/disk-image-create: 146: [: aa: unexpected operator

Change-Id: I619d7bf2e4b56885d629019b80b249bc80b8f43e

bin/disk-image-create

@@ -143,7 +143,7 @@ if [ "$IS_RAMDISK" == "1" ]; then
 fi
 
 # If no elements are specified theres no way we can succeed
-if [ -z "$@" ]; then
+if [ -z "$*" ]; then
     echo "ERROR: At least one distribution root element must be specified"
     exit 1
 fi