It make no sense to have a kernel which do not have any SYSINIT entries
or contains only one placeholder entry but not to be executed. It is
unlikely that we will allow such kind of setup in future.
As for kernel modules, at least one sysinit entry is declared with
DECLARE_MODULE[_XXX]. Also linker_file_lookup_set() is invoked to
get sys[un]init_set prior to run sys[un]init tasks, so no harm when
trying to load a invalid module which has no / an empty sys[un]init_set.
Retire SI_SUB_DUMMY now to make the logic simpler and easier to maintain.
MFC after: 2 weeks