postinst: allow any system to skip kexec, not just Xen

The number of platforms that have to disable kexec has grown. Instead of
explicitly detecting one, Xen, test directly for whether the system is
configured to use boot_kernel or boot directly into the latest version.

Required for https://github.com/coreos/bugs/issues/75 and https://github.com/coreos/bugs/issues/68