update command copying image for linux

[panosnethood]

Message from commonslab.gr:

The dd command supports progress monitor with the command parameter "status=progress"

There are various places where we have this guideline and it would be nice to update it

[hellekin]

You may also send a USR1 signal to dd to show progress. E.g., from the dd documentation:

# Output stats every second.
while kill -s USR1 $pid 2>/dev/null; do sleep 1; done