core: in bus_init_system() make sure we setup the system bus even if we inherit the API bus

This corrects the control flow: when we reuse the API bus as system bus,
let's definitely invoke bus_init_system() too, so that it is called
regardless how we acquired the bus object.

(Note that this doesn't actually change anything, as we only inherit the
bus like this in system mode, and bus_init_system() doesn't do anything
in system bus, besides writing a log message)