Putting all device images into a same directory when --repackage is given.

moves all image files into one tmp directory and then repackage them,
so relative paths to the fetched files would not be added to archive
file, only files themselves.

Test: flash --flasher_type=custom --flasher_path=<path/to/custom/flasher> --repackage=tar.md5
Bug: 71972164
Change-Id: Id19a7ef7b6bb44b4e59f61bda11536554e10fd23