Use :- instead of - for empty environment variables

We set the variables to empty when we start processing, so ${FOO-foo}
will never print "foo". We need to use ${FOO:-foo}.

Change-Id: I00c28edb10d8eaa09df689905a302b576b246806
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>