the attribute 'ScreenOrientation' is only available for dedicated microcontrollers, where the hardware is not able to make a rotation.
In case of Raspberry Pi the underlying graphics API is OpenGL ES 2.0. If you want to define the orientation of the screen (0° - 90° - 180° - 270°), you can just create the UI as it will appear to the user - and let make OpenGL the final rotation on the screen. For doing this, you can rotate the viewport of your application.
Please have a look into your main.c file. There you will find the following initialization of the viewport:
viewport = EwInitViewport( EwScreenSize, EwNewRect( 0, 0, w, h ), 0, 255,
&framebuffer, eglDisplay, eglSurface, ViewportProc );
The third parameter is the orientation hint of the viewport - by default this is set to 0. The meaning of this parameter depends on the target system. For example, OpenGL targets are using this parameter to determine the screen rotation in degrees.