bootstash: Do not provide a default address for all

A valid memory location to stash bootstage information at will be
architecture dependent. Move the existing defaults to the main Kconfig
file for this option and set 0x0 as the default only for sandbox.

Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Rini <trini@konsulko.com>