Changed serial console setup

The previous method only configured serial console for Ubuntu or
Fedora, I noticed this wasn't working when I built RedHat 7.
After reading about how grub-mkconfig works, I realized that we
shouldn't be using sed to modify grub.cfg, and set up the proper
variables in /etc/default/grub

Change-Id: I02b73dc5074cb48a716849cb8a8a9b2f054bea31
Merge "Changed serial console setup"