Allow for alternative grub config file

The grub config file passed into sed for the tty purge doesn't exist on
fedora, causing image builds to fail.

can't read /boot/grub/grub.cfg: No such file or directory

Change-Id: I015abba81bf67e2ed7c5a2876adeaef641a6cc3c
Merge "Allow for alternative grub config file"