build: Enable super image build rules depending on single super block device

* The current logic does not suit for multiple block devices case,
  it generates split images hence output dir shall be specified
* Multiple block devices is possible with non-RDAP too, and it could
  work for RDAP with single block device, so it's incorrect to depend
  on RDAP flag

Change-Id: I8563b96b35351ef0a3b40ae172c2e904ee57f85e