Allow modifying a kernel that differs to what runs

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.
Merge pull request #15 from gsandie/kernel_differ_to_initramfs

Allow modifying a kernel that differs to the one running