x86: zimage: Use a state struct to hold the state

x86: zimage: Use a state struct to hold the state

At present the 'zboot' command does everything in one go. It would be
better if it supported sub-commands like bootm, so it is possible to
examine what will be booted before actually booting it.

In preparation for this, move the 'state' of the command into a struct.
This will allow it to be shared among multiple functions in this file.

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>