Export unbound variable DIB_RHSM_USER

If 'DIB_RHSM_USER' isn't set, the image build with rhel
element fails because of check 'set -u'. So, we should export
'DIB_RHSM_USER' in script to avoid error during build

Change-Id: Ic63a20e3d91a25bbe804440a5919b035489c60a3
Closes-bug: #1318717

elements/rhel/finalise.d/60-rhsm-unregister

@@ -3,6 +3,8 @@
 set -eu
 set -o pipefail
 
+export DIB_RHSM_USER=${DIB_RHSM_USER:-}
+
 if [ -n "$DIB_RHSM_USER" ] && [ -n "$DIB_RHSM_PASSWORD" ]; then
   if [[ "$DIB_REG_TYPE" == "rhn" ]]; then
     rm -rf /etc/sysconfig/rhn/systemid