Introduce volume_{present,removable} and fix invalid calls in apps/

The code was trying to probe for volume presence by calling drive layer
with volume index. It is a miracle it get unnoticed so far. Introduce
proper volume probing using the vol->drive map in the disk layer.

Change-Id: I463a5bcc8170f007cad049536094207d2ba3c6fc
Reviewed-on: http://gerrit.rockbox.org/669
Reviewed-by: Amaury Pouly <amaury.pouly@gmail.com>
3 files changed