Experimental branch. When UPC requires a weight, set the UPC in
session SNR. When a weight is registered via DisplayLib::scaledisplaymsg,
the UPC is returned as part of the input. This way, I can "scan"
the UPC as soon a weight gets set. The response should be quicker
than retry-ing the UPC at set intervals especially when session
contention/locking is taken into account.
BUGFIX: verify array index exists
Merge pull request #129 from CORE-POS/wait-for-scale

FYI: Wait for scale change