spl: Add an SPL_HAVE_INIT_STACK option

At present there is a hex value SPL_STACK which both determines whether
SPL has its own initial stack and the hex value of that stack.

Split off the former into SPL_HAVE_INIT_STACK with SPL_STACK depending
on that and only providing the latter.

Signed-off-by: Simon Glass <sjg@chromium.org>
[trini: Resync defconfig files]
246 files changed