bootindex: add check bootindex function

Determine whether a given bootindex exists or not.
If exists, we report an error.

Signed-off-by: Gonglei <arei.gonglei@huawei.com>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
