bloblist: Adjust API to align in powers of 2

The updated bloblist structure stores the alignment as a power-of-two
value in its structures. Adjust the API to use this, to avoid needing to
calling ilog2().
Update the bloblist alignment from 16 bytes to 8 bytes.
Drop a stale comment while we are here.

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>