Use default image name in webosvbox script

Open-webOS-DCO-1.0-Signed-off-by: Keith Derrick <keith.derrick@palm.com>

Change-Id: I1209185e11badb0a95aa16170b0524d5d2e614e7
Reviewed-on: http://g2g.palm.com/1363
Reviewed-by: Keith Derrick <keith.derrick@palm.com>
Tested-by: Keith Derrick <keith.derrick@palm.com>