[update] Greatly simplify sdcard handling

* Use /media/sdcard instead of /run/user/<uid>/media/sdcard
* the new mount path is available during early boot so no systemd services needed
* Don't remove mount dirs so that tracker can keep track of volume root nodes
* Not removing mount dirs also avoids a rare tracker race condition leading to a crash
Merge pull request #10 from nemomobile/simplify

[sd-utils] Greatly simplify sdcard handling