Allow child binaries to be ERIS URNs or otherwise arbitrary strings.
Add an option to genode.core.children and genode.init.children for specifying a Nic driver for the exclusive use of the child for which it is added. This mechanism is to support the nic_router but it is reusable for arbitrary children.
The "config.genode.init.children" option is a superset of the "config.genode.core.children" option, so refactor both into helper functions.