bootstage: Copy bootstage strings post-relocation

bootstage: Copy bootstage strings post-relocation

Any pointers to name strings that were passed to bootstage_mark_name()
pre-relocation should be copied post-relocation so that they don't get
trashed as the original location of U-Boot is re-used for other
purposes.

This change introduces a new API call that should be called from
board_init_r() after malloc has been initted on any board that uses
bootstage.

Signed-off-by: Doug Anderson <dianders@chromium.org>
Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Simon Glass <sjg@chromium.org>