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]
diff --git a/arch/riscv/cpu/start.S b/arch/riscv/cpu/start.S
index 3f78932..c8526dd 100644
--- a/arch/riscv/cpu/start.S
+++ b/arch/riscv/cpu/start.S
@@ -90,7 +90,7 @@
  * Set stackpointer in internal/ex RAM to call board_init_f
  */
 call_board_init_f:
-#if defined(CONFIG_XPL_BUILD) && defined(CONFIG_SPL_STACK)
+#if defined(CONFIG_XPL_BUILD) && defined(CONFIG_SPL_HAVE_INIT_STACK)
 	li	t0, CONFIG_SPL_STACK
 #else
 	li	t0, SYS_INIT_SP_ADDR