config_unpack didn't changed the semantics it was just introduced to remove
duplicate code. 
But i think the current behavior is a "feature" as you could specify
config=a.tbz myconfig=/dev/sda1 and it would unpack a.tbz from /dev/sda1. 
Otherwise, if there is a config file on the live-image _and_ on the specified
disk, the config file would be unpacked twice but this could be also used as a
feature.
For example: the config file on the live-image is for general purpose (root 
password, authorized keys) and the config file on the block device contains only
the ssh host key for the specific machine.

I think it is not worth to change the code as issue 715 and the patches look
promising.