kernel-grub.bbclass: add a method to install/update for bzImage
While installing a rpm to update kernel on a deployed target, it will update
the boot area and the boot menu with the kernel as the priority but allow
you to fall back to the original kernel as well.
- In kernel-image's preinstall scriptlet, it backs up original kernel to avoid
probable confliction with the new one.
- In kernel-image's postinstall scriptlet, it modify grub's config file to
updates the new kernel as the boot priority.
