GRUB in CentOS 7

The way grub entries are defined in centos 7 has changed.
Instead of using grub.conf, grub2 now uses /boot/grub2/grub.cfg, with content generated from scripts in /etc/grub.d/

[root@localhost grub.d]# ll
total 64
-rwxr-xr-x. 1 root root  8698 Jun 30  2014 00_header
-rwxr-xr-x. 1 root root  9517 Jun 30  2014 10_linux
-rwxr-xr-x. 1 root root 10275 Jun 30  2014 20_linux_xen
-rwxr-xr-x. 1 root root  2559 Jun 30  2014 20_ppc_terminfo
-rwxr-xr-x. 1 root root 11110 Jun 30  2014 30_os-prober
-rwxr-xr-x. 1 root root   380 Mar 14 03:30 40_custom
-rwxr-xr-x. 1 root root   216 Jun 30  2014 41_custom
-rw-r--r--. 1 root root   483 Jun 30  2014 README

the file 40_custom is what you will need to modify.

For my custom kernel, the fie looked like this:

[root@localhost grub.d]# cat 40_custom
#!/bin/sh
exec tail -n +3 $0
# This file provides an easy way to add custom menu entries.  Simply type the
# menu entries you want to add after this comment.  Be careful not to change
# the 'exec tail' line above.
menuentry "Robbie's custom kernel" {
        set root=UUID=91df2431-f383-4d9d-873f-8df5bb2e00fa
        linux=/vmlinuz-3.10.0-123.el7.robbie.x86_64
        initrd=/initrd-plymouth.img
        }

Then, if you wish to edit the default boot entry, you can modify /etc/default/grub with the boot entry you desire:

GRUB_DEFAULT="Robbie's custom kernel"