Fixed search Fuel ISO in vbox scripts

Fixed relative paths when are invoked other vbox scripts.
If we have another config.sh which is located on a different path
in the variable PATH, we have an error.

Change-Id: I1dfadebac4f76d18f85ca396a202d3550948042d
Closes-Bug: #1393557
Merge "Fixed search Fuel ISO in vbox scripts" into stable/6.0