Fix missing export of $ARCH
Fix missing export of $ARCH

So that elements can have access to the variable.

Change-Id: I9a63160ef891e3a3a3c6c90856e2539250c8b807

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