Small fix to contrib VirtualBox script

Fixed VirtualBox script to create final VDI file at correct location.
Removed the file extension from VirtualBox script.
Merge pull request #220 from skarllot/master

Small fix to contrib VirtualBox script
fix(setup_board): Allow make.conf.user to be per-board

Previously this would symlink to a global make.conf.user making it
impossible to apply changes to a board but not the SDK.
Merge pull request #223 from marineam/user

fix(setup_board): Allow make.conf.user to be per-board