Update dib script usage
Update dib script usage

Update manual dib mounting instructions to match changes made in the
tools/mount-image.sh script (I0deba1907f390f0d308ae6c8ed5448afbbf7688a)
and add a reference to the script itself in the documentation.

Change-Id: Ic72bf05f21cecb8bdff36956f3635c2f504c4dff

coolsvap/project-config,dongwenjuan/project-config,citrix-openstack/project-config,dongwenjuan/project-config,openstack-infra/project-config,noorul/os-project-config,Tesora/tesora-project-config,citrix-openstack/project-config,noorul/os-project-config,anbangr/osci-project-config,anbangr/osci-project-config,Tesora/tesora-project-config,coolsvap/project-config,openstack-infra/project-config