Enhance liblights to support touchkey backlight

Adds the necessary function to control capacitive key backlight from userspace

Set BOARD_BUTTON_LIGHTS_PATH in BoardConfig to the relevant device node path to enable this

Used by Infuse4g at least - May be applicable to other devices? (epicmtd has a similar function in their liblights)

Change-Id: I2bebd6a8b2dc7624c42c78e9862246f10725a000