scripts: gen_defines: re-work write_bus() with augmented nodes

This too is an attempt to reimplement the previous behavior exactly,
modulo the order in which things are defined.

This is the last function which is calling into the previous
implementation's out_node and node_*_alias() functions, so these can
be removed now.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>