file should be checked for MD5 even if it is already present

When a file is already present, for instance a hotfix, and it is partly downloaded, no check is made and thus the file is not downloaded again.

This happens at least when the connection is lost or when the build is aborted.

1. retrieveHFPackage should be modified to store the MD5 in a file once downloaded from connect.
2. a MD5 check should be performed whether the file was downloaded or was already present