bloblist: Handle alignment with a void entry

Rather than setting the alignment using the header size, add an entirely
new entry to cover the gap left by the alignment.

Signed-off-by: Simon Glass <sjg@chromium.org>
Co-developed-by: Raymond Mao <raymond.mao@linaro.org>
Signed-off-by: Raymond Mao <raymond.mao@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>