vim /etc/default/grub
修改timeout =2
vim /boot/grub2/grub.cfg
修改centos/grub.cfg中的以下部分内容后重启CentOS7，OK！
if [ x$feature_timeout_style = xy ] ; then
set timeout_style=menu
set timeout=2
# Fallback normal timeout code in case the timeout_style feature is
# unavailable.
else
set timeout=2
fi