tslib: Derive default device value from TSLIB_TSDEVICE environment variable

Most devices that use tslib have the device path defined by
the TSLIB_TSDEVICE env. variable.

Change-Id: I89fc17cb2c7fba7741de2fd348aacd608cb21a2a
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>