Use readlink to get script path
Use readlink to get script path

So we can support usage:
ln -s ~/source/.../disk-image-create ~/bin/

Change-Id: Id54b613a247679b98cef3e9a6f37cd93155d029e

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