To set GRUB timeout to 5 seconds, use this command.
sed -i.bak 's/timeout=5/timeout=5/g' /boot/grub/grub.cfg
To disable timeout (first option will be selected automatically) use this command.
sed -i.bak 's/timeout=5/timeout=0/g' /boot/grub/grub.cfg
To halt boot menu until any option is selected, use this one.
sed -i.bak 's/timeout=5/timeout=-1/g' /boot/grub/grub.cfg
If you want to disable creating backup, remove .bak from sed's option. For examlple,
sed -i 's/timeout=5/timeout=1/g' /boot/grub/grub.cfg