Handle config files
We need to handle config files specially. This means checking their hashes (note that newer opkg seems to use sha1 instead of md5), and don't overwrite if they are changed. Also, don't delete them if they are not changed.
Think what happens if we remove a package, leave a modified config file in place and a new package comes with the same config file. Should we overwrite it? If not, how do we know it has been modified and has been a config file, if no package currently claims it? We should probably be able to handle this at least during a single transaction.