boot: Reuse existing core object if any
boot: Reuse existing core object if any

This is to allow for externally created core objects to be reused rather
than trying to create a new one (for embedding POX).

MurphyMc/pox,pthien92/sdn,MurphyMc/pox,pthien92/sdn,noxrepo/pox,pthien92/sdn,MurphyMc/pox,noxrepo/pox,pthien92/sdn,MurphyMc/pox,pthien92/sdn,noxrepo/pox,MurphyMc/pox,noxrepo/pox