Configure Ubuntu 18.04 with grub2 to activate serial console

Edit the file /etc/default/grub

1. Change GRUB terminal to console and ttyS0. This will provide one GRUB to a monitor display and serial console.
2. Change linux kernel console to tty1 and ttyS0. This setting will be taken over to userland, and there will be two login prompt for tty1 and ttyS0.

GRUB_CMDLINE_LINUX="console=tty1 console=ttyS0,115200"
GRUB_TERMINAL="console serial"
GRUB_SERIAL_COMMAND="serial --speed=115200 --unit=0 --word=8 --parity=no --stop=1"