Allow modifying a kernel that differs to the one running

Sometimes during unattended installs you might be running an installer
kernel that is different to the one one the installed machine. This
change first checks to see if the an initramfs file that matches the
running kernel exists, and if not will look for one in /boot.

This makes an assumption that there is only a single kernel and thus
single initrd in the boot directory.

Note that this support has only been added to CentOS 6 as that's the
platform I'm working with.