bootstage: Add feature to stash/unstash bootstage info

It is useful to be able to write the bootstage information to memory for
use by a later utility, or the Linux kernel. Provide a function to do
this as well as a function to read bootstage information back and incorporate
it into the current table.

This also makes it possible for U-Boot to chain to another U-Boot and pass
on its bootstage information.

Signed-off-by: Simon Glass <sjg@chromium.org>