Hartid and cpuindex are not the same thing. Hartid is needed regardless
of SMP, for external interrupt handling etc.
SMP needs cpuindex which might not be index == hartid, so both are
needed. IMO it is clearer to provide separate API for both.
Currently the implementation of up_cpu_index is done a bit lazily,
because it assumes hartid == cpu index, but this is not 100% accurate,
so it is still missing some logic.