bloblist: refactor of bloblist_reloc()

The current bloblist pointer and size can be retrieved from global
data, so we don't need to pass them from the function arguments.
This change also help to remove all external access of gd->bloblist
outside of bloblist module.

Signed-off-by: Raymond Mao <raymond.mao@linaro.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>