bootstd: Add support for updating elements of the cmdline

bootstd: Add support for updating elements of the cmdline

Add a bootflow command to update the command line more easily. This allows
changing a particular parameter rather than editing a very long strings.
It is also easier to handle with scripting.

The new 'bootflow cmdline' command allows getting and setting single
parameters.

Fix up the example output while we are here, since there are a few new
items.

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