bootstd: Allow storing x86 setup information

bootstd: Allow storing x86 setup information

On x86 boards Linux uses a block of binary data to provide information
about the command line, memory map, etc. Provide a way to store this in
the bootflow so it can be passed on to the OS.

No attempt is made to generalise the code, since other archs don't need
this information. The field is present always, though, to avoid needing
accessors or #ifdefs when building code on other archs.

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