display_options: Refactor to allow obtaining the banner

Move the display options code into a separate function so that the U-Boot
banner can be obtained from other code. Adjust the 'version' command to
use it.

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