spl: Use CONFIG_VAL() to obtain the SPL stack

spl: Use CONFIG_VAL() to obtain the SPL stack

Now that we have the same option for SPL and TPL, simplify the logic for
determining the initial stack.

Note that this changes behaviour as current SPL_STACK is a fallback for
TPL. However, that was likely unintended and can be handled with Kconfig
defaults if needed.

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