x86: zboot: Allow setting a separate setup base address

x86: zboot: Allow setting a separate setup base address

At present the setup block is always obtained from the image
automatically. In some cases it can be useful to use a setup block
obtained elsewhere, e.g. if the image has already been unpacked. Add an
argument to support this and update the logic to use it if provided.

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: adjust maxargs to 7 for 'zboot start']
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>