Actually power off the handheld with the sleep timer and idle poweroff functions instead of closing rockbox.
This commit also moves setting system volume to max from the launch script to system-sdl.c so it works when running rockbox via instant play.
Change-Id: I174b67c6abef451faa05f4a8f52b8d9eeed98a22
Instant play is a Funkey OS feature that allows it to relaunch at boot the last program it was running before shutting down, which means that if the handheld is powered off (holding the power button) while rockbox is running the next time it's powered on rockbox will launch at boot.
This commit also handles system brightness/volume in rockbox itself instead of the launch script, so the values are properly reset when powering off the handheld while rockbox is running.
Change-Id: Ie1adbf71069aeed5fbf6670971718a2f718716a3
