Are you familiar with /etc/rc.d and rc scripts? Have a squiz
here I'm not sure what would have to be different on a slack system but in Section 3 there is a script I'd name rc.kodi, put it on /etc/rc.d and chmod it +x. Other systms tend to complicate things and some or all of above Section 3 might be required for system, dunno, I've never layed with XBMC a.k.a Kodi.