Add documentation for the log system.
Change-Id: I1ad874d516ea2c140bf37d5ae695479b087620fa
Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Signed-off-by: Kever Yang <kever.yang@rock-chips.com>
(cherry picked from commit 1d0f30a8e080fd490ec4df50d5ccfc742e771ca0)
Simon Glass