bloblist: Checksum the entire bloblist

Use a sinple 8-bit checksum for bloblist, as specified by the spec
version 0.9.
Spec v0.9 specifies that the entire bloblist area is checksummed,
including unused portions. Update the code to follow this.

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: Ilias Apalodimas <ilias.apalodimas@linaro.org>