x86: zboot: Allow overriding the command line

x86: zboot: Allow overriding the command line

When booting Chrome OS images the command line is stored separately
from the kernel. Add a way to specify this address so that images boot
correctly.

Also add comments to the zimage.h header.

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