Clip: add backlight and buttonlight code

Revive lcd_enable() as well, and use it in _backlight_on/off()
Don't update the lcd framebuffer if the display is off

git-svn-id: svn://svn.rockbox.org/rockbox/trunk@18952 a1c6a512-1295-4272-9138-f99709370657
34 files changed