Use QDirListing in the Bootstrap build instead of QDirIterator.
Drive-by changes:
- more const variables
- use emplace_back() instead of append() where appropriate
Change-Id: Ie1f0d03856e557c4bfabfff38a87edc7da86d091
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>