Update webosvbox for new repository layout

:Release Notes:
The webosvbox script now recognizes a build-webos directory
with the latest layout.

:Detailed Notes:
Recent changes in layer names etc, broke the sanity check to
ensure that webosvbox was being launched from the top directory
of a build tree.

Modify the list of sentinel files to contain something more generic.

:Testing Performed:
Used the script to create, launch, stop, and ssh into virtual box
images.

:QA Notes:

:Issues Addressed:
[CT-206] Restore the virtualbox image to a usable state

Open-webOS-DCO-1.0-Signed-off-by: Keith Derrick <keith.derrick@lge.com>
Change-Id: Ibe18f03f6685c282f60608646c515283e32097ef
Originally-Reviewed-on: https://gpro.palm.com/55644
Reviewed-on: https://g2g.palm.com/6330
Tested-by: Martin Jansa <Martin.Jansa@lge.com>
Reviewed-by: Martin Jansa <Martin.Jansa@lge.com>
Reviewed-by: Keith Derrick <keith.derrick@lge.com>
Reviewed-by: Susan Montooth <susan.montooth@lge.com>