Straigten-out lcd sleeping on Gigabeat F/X. Add a service function to backlight.c...

Straigten-out lcd sleeping on Gigabeat F/X. Add a service function to backlight.c to handle lcd sleep timer. Make HAVE_LCD_SLEEP useable without a setting and use HAVE_LCD_SLEEP_SETTING when a setting is available in addition to HCD_HAVE_SLEEP. If a setting isn't used, the target must define the timeout to be used in the config.

git-svn-id: svn://svn.rockbox.org/rockbox/trunk@17505 a1c6a512-1295-4272-9138-f99709370657