minui: Allow devices to blank/unblank using LCD backlight

* Some devices rely on the LCD backlight to blank/unblank the display,
  support them by allowing a custom LCD backlight path to be used.

Change-Id: I8406b4b55cd7a2afc4f8f8ba65de2c53b949489d
Merge "minui: Allow devices to blank/unblank using LCD backlight" into cm-10.2
Merge branch 'cm-10.2' into martincz