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/x530_defconfig b/configs/x530_defconfig
index fa385fc..ac418da 100644
--- a/configs/x530_defconfig
+++ b/configs/x530_defconfig
@@ -32,6 +32,8 @@
 CONFIG_SPL_BSS_MAX_SIZE=0x4000
 CONFIG_SPL_BOARD_INIT=y
 CONFIG_SPL_SYS_MALLOC_SIMPLE=y
+# CONFIG_SPL_SHARES_INIT_SP_ADDR is not set
+CONFIG_SPL_STACK=0x4002c000
 CONFIG_SPL_WATCHDOG=y
 CONFIG_SYS_CBSIZE=256
 CONFIG_SYS_PBSIZE=276