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/controlcenterdc_defconfig b/configs/controlcenterdc_defconfig
index fd8cbe5..9b53809 100644
--- a/configs/controlcenterdc_defconfig
+++ b/configs/controlcenterdc_defconfig
@@ -38,6 +38,8 @@
 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=0x40031000
 CONFIG_SPL_I2C=y
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_MAXARGS=32