Found the culprit in arch/arm/include/asm/fb.h
Fix has been pushed to github. X11 with fbturbo video driver confirmed working and rotated, xrandr is convinced my HDMI monitor is 720x1280 ;-)
Matthijs
PS it's kinda weird how much faster xterm is than the framebuffer console... I mean I know I broke "fast ywrap" but xterm is clearly doing fine without... oh well