gen: add support for null shim
gen: add support for null shim

This is useful to create L3/L2 topologies without running any RINA
software on it.

vmaffione/rlite,vmaffione/rlite,autrimpo/rlite,autrimpo/rlite,autrimpo/rlite,vmaffione/rlite,vmaffione/rlite,autrimpo/rlite