Convert CONFIG_SPL_STACK to Kconfig

This converts the following to Kconfig:
   CONFIG_SPL_STACK

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/harmony_defconfig b/configs/harmony_defconfig
index 6497d41..50a2107 100644
--- a/configs/harmony_defconfig
+++ b/configs/harmony_defconfig
@@ -14,6 +14,8 @@
 CONFIG_SYS_STDIO_DEREGISTER=y
 CONFIG_SPL_FOOTPRINT_LIMIT=y
 CONFIG_SPL_MAX_FOOTPRINT=0x8000
+# CONFIG_SPL_SHARES_INIT_SP_ADDR is not set
+CONFIG_SPL_STACK=0xffffc
 CONFIG_SYS_PROMPT="Tegra20 (Harmony) # "
 CONFIG_SYS_MAXARGS=64
 CONFIG_SYS_PBSIZE=2085