kdumpctl: force rebuild in case of dynamic system modification
    
    There could be some dynamic system modification, which may affect kdump
    kernel boot process. In such situation initramfs must be rebuilt on the
    basis of changes.
    Since most of these checking methods will use information from
    TARGET_INITRD, therefore check its existence in common code.
    
    Signed-off-by: Pratyush Anand <panand@redhat.com>
    Acked-by: Xunlei Pang <xlpang@redhat.com>
    Acked-by: Baoquan He <xlpang@redhat.com>
    Acked-by: Dave Young <dyoung@redhat.com>