backlight control on Luna: switch PWL instead of LEDB This change is preliminary toward upcoming rework of backlight control logic in our UI firmware. LEDB does not exist on Tango-based platforms (it is not brought out on Tango modules), thus turning it on and off produces absolutely no effect beyond making L1 disable deep sleep when LEDB is turned on. However, both iWOW DSK and our upcoming FC Caramel2 boards have a PWL LED, so let's switch that LED on and off to indicate the state of the UI firmware's backlight control. Note that we are NOT switching the actual Luna LCD backlight here, even though it is trivially controlled with a GPIO. The reason for this seemingly strange choice is that we don't want to turn this development board LCD backlight off until we bring the higher-level backlight control logic up to par, including new logic to "swallow" the first keypress that turns on the darkened LCD.