I'm trying to use this runtime config.txt parameter under Raspbian Stretch and an external LED monitor on the HDMI port to no effect. It's hard to tell from posts here and elsewhere if this setting only effects the Pi display or external displays as well.
will turn the HDMI display upside-down whereas
will turn the directly attached touchscreen display (not HDMI) upside-down. I've seen reports that say this may not work with certain touchscreen display models though.