extract_files: Add support for specifying blob sections

Sometimes the need arises to update one specific subset of blobs
while holding others steady. Introduce a way to specify the "section"
of blobs to update (obviously requires the proprietary-files to be
properly split up first)

Requires additional changes in device tree extract_files.sh to support
passing the additional arguments

Change-Id: I98feab56f8bfade2818b1b5264e019440d4c57c5