bootstage: Avoid conflicts between stash/unstash

At present there is a single shared address for bootstage data in both
TPL and SPL. If SPL unstashs TPL bootstage info and then stashes it again
to pass it to U-Boot, the new stash overwrites the strings of the old
stash.

Fix this by duplicating the strings into the malloc() region. This should
be a small code. Fix the header-file order at the same time.

This problem doesn't happen at the next stage (SPL->U-Boot) since U-Boot
relocates the boostage data.

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