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: Tom Rini <trini@konsulko.com>
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Changes in v2:
- Seeing that BOOTSTAGE_STASH_ADDR did not depend on BOOTSTAGE_STASH in
  turn lead to discovering that (minor) BOOTSTAGE_STASH_SIZE was also
  missing a depends on line and then that a number of places built code
  with BOOTSTAGE_STASH_ADDR=0x0 as a compiles-but-likely-fails option.
  Rework a number of spots to guard usage around BOOTSTAGE_STASH being
  enabled.