releasetools: Don't assert unmount.

-If /system is already unmounted the assert fails. We don't
really care as long as its unmounted so.. no assert!
overlay: Enable moving apps to "real" sdcard

-Made possible by the recent addtion of this patch
https://github.com/CarbonDev/android_packages_apps_Settings/commit/030140194a3f10bff70f03896d4e1daa6a256548