export DIB_ROOT_LABEL to make it global

It is needed by the debian element in the root.d phase. (enviroment.d variables are not available after the base image is built)

Change-Id: Idb8f9795a619687569bcb0de774c87333d11b292
Closes-Bug: #1384103
Merge "export DIB_ROOT_LABEL to make it global"