/etc/default/grub:

GRUB_CMDLINE_LINUX="video=off elevator=deadline console=tty0 console=ttyS0,115200"
GRUB_TERMINAL=serial
GRUB_SERIAL_COMMAND="serial --unit=0 --speed=115200 --stop=1"

/etc/inittab:

0:2345:respawn:/sbin/agetty -8 ttyS0 115200 vt100