bitbake.conf: Introduce variables to specify machine's display properties.

bitbake.conf: Introduce variables to specify machine's display properties.
* Per RFC on the mailing list.
* Following variables added:
MACHINE_DISPLAY_WIDTH_PIXELS - width in pixels in hardware orientation
MACHINE_DISPLAY_HEIGHT_PIXELS - height in pixels in hardware orientation
MACHINE_DISPLAY_ORIENTATION - number of degrees hardware display orientation must
  be rotated to be in "default natural use" orientation.
MACHINE_DISPLAY_BPP - bits per pixel
* Also, deprecate GUI_MACHINE_CLASS and replace with MACHINE_GUI_CLASS.