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/evb-px30_defconfig b/configs/evb-px30_defconfig
index 019e942..7b1138e 100644
--- a/configs/evb-px30_defconfig
+++ b/configs/evb-px30_defconfig
@@ -35,6 +35,8 @@
 CONFIG_SPL_BSS_MAX_SIZE=0x4000
 CONFIG_SPL_BOOTROM_SUPPORT=y
 # CONFIG_SPL_RAW_IMAGE_SUPPORT is not set
+# CONFIG_SPL_SHARES_INIT_SP_ADDR is not set
+CONFIG_SPL_STACK=0x400000
 CONFIG_SPL_STACK_R=y
 # CONFIG_TPL_BANNER_PRINT is not set
 CONFIG_SPL_ATF=y