If the prop persist.twrp.rotation is set to 0, 90, 180, or 270,
that will be used. If the prop is not set, the existing rotation
board config flags will be honored.
Note: This flag will only rotate graphics. If a set of devices
needs differing touch rotation, that support will need added
separately.
Change-Id: I6996c07bc0ae7bb8555b3d480b2917a17a365eb3