video: Support showing the U-Boot logo

Show the U-Boot logo by default. This is only 7KB in size so seems like
a useful default for boards that enable a display.

If SPLASH_SCREEN is enabled, it is not enabled by default, so as not to
conflict with that feature.

Also disable it for tests, since we don't want to complicate the output.

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