x86: zboot: Set up a sub-command structure

x86: zboot: Set up a sub-command structure

Add subcommands to zboot. At present there is only one called 'start'
which does the whole boot. It is the default command so is optional.

Change the 's' string variable to const while we are here.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Wolfgang Wallner <wolfgang.wallner@br-automation.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
[bmeng: reduce maxargs to 6 of 'zboot start' subcommand]
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>