The best I can do is give you a hint since I haven't done auto-login on serial.
inittab was part of sysvinit, the latest versions of debianDebian and Raspbian (Jessieviz., Jessie) and therefore Raspbian use systemd which doesn't have inittab. Instead it uses systemctl to control the things that inittab did before.
do a man on systemctl to maybe find what you're after.