Rotate Plymouth-based UC22 splash to portrait

Continuing the discussion from Psplash for custom pi gadget device as a new topic since my follow-up question does not involve psplash:

Users of my appliance image may attach a wide variety of off-the-shelf HDMI displays in both upright and normal (landscape) positions, so I cannot rely on EDID to provide the current orientation. ubuntu-frame gets the proper display layout via the gadget snap, i.e. only the splash screen has the wrong orientation.