packaging: cleanup() did not read the value of $CLEANUP

When the original author put CLEANUP in a conditional statement he was probably
willing to use it to control the "if". This change tries to restore that
behaviour: the "rm" clause is triggered if and only if CLEANUP is defined and
not empty.

contrib/packaging/builddeb

@@ -15,7 +15,7 b' CODENAME=`lsb_release -cs 2> /dev/null |'
 DEBFLAGS=-b
 
 cleanup() {
-    if [ '$CLEANUP' ]; then
+    if [ "$CLEANUP" ]; then
         rm -r "$PWD/debian";
     fi
 }