Screen Power Manager Writer

The code for this functionality is incomplete, but necessary if you want to run the e39-rpi without also wiring it inline with a Euro TV Tuner module or an mk4 navigation unit.

Both the Euro TV Tuner module and the MK4 navigation computer send the messages to turn on and off the BMBT LCD screen. Without these messages, the screen is never turned on and the brightness is never set. Additionally, the aspect ratio of the screen can be configured, for both narrow and wide-screen BMBT options.

The code for this is based on research done in Wilhelm-docs for screen control.