Make xbmc-data directory so not case sensitive

Document a few variables
Have dropbear put SSH server keys in /tmp/userdata/etc/dropbear so they persist

This will cause the keys to persist across firmware updates and works regardless of whether you have data on sdcard or internal storage.
There is still a fallback, the S10setup script still makes keys in /etc/dropbear so if something is wrong with /root still will start SSH
Enable cache for copying to memory, card then disable it for safety