buildman: Fix bloat option when 'new' only drops functions

In the case where a new build only decreases sizes and does not increase
any size we still want to report what functions have been dropped when
doing a bloat comparison.

Cc: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>