Allow to disable vmcore plugin

On AArch64 architecture there is no kexec/kdump support yet. Disabling
vmcore plugin allows to install Fedora there.

Signed-off-by: Marcin Juszkiewicz <mjuszkiewicz@redhat.com>
spec: disable addon-vmcore on aarch64

Signed-off-by: Jakub Filak <jfilak@redhat.com>
Merge branch 'hrw-master'