avoid failure if /lib/firmware doesn't exist
avoid failure if /lib/firmware doesn't exist

On Debian, /lib/firmware is provided by optional packages like
firmware-iwlwifi or firmware-linux-free. That's why this directory
may not exist.
This change ensure the directory exist before trying to copy it
content.

Change-Id: Icf9eedf4148141c92e9171f16ae744b88a8d8519

NaohiroTamura/diskimage-builder,takahashinobuyuki/diskimage-builder,rdo-management/diskimage-builder,NaohiroTamura/diskimage-builder,varunarya10/diskimage-builder,zioproto/diskimage-builder,CSC-IT-Center-for-Science/diskimage-builder,eantyshev/diskimage-builder,eantyshev/diskimage-builder,switch-ch/diskimage-builder,CSC-IT-Center-for-Science/diskimage-builder,zioproto/diskimage-builder,switch-ch/diskimage-builder,openstack/diskimage-builder,rdo-management/diskimage-builder,takahashinobuyuki/diskimage-builder,JioCloud/diskimage-builder,openstack/diskimage-builder,JioCloud/diskimage-builder,varunarya10/diskimage-builder