At present this just outputs the message. Update it to output whatever the
format requests.

Signed-off-by: Simon Glass <sjg@chromium.org>

It is useful to display log messages on the console. Add a simple driver
to handle this.

Note that this driver outputs to the console, which may be serial or
video. It does not specifically select serial output.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>