Psplash for custom pi gadget device

@abeato @ogra How do I rotate the Plymouth splash? I found this discussion pointing me to the kernel command line video=HDMI-A-1:panel_orientation=right_side_up (for portrait orientation), but that does not seem to do the trick. Tested on an RPi 4, the intermittent blinking cursor was displayed upright. My cmdline addition is in my gadget’s cmdline.extra file. Maybe these options are not passed to the initramfs-based Plymouth splash?