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/Cubietruck_plus_defconfig b/configs/Cubietruck_plus_defconfig
index cbd77cf..8119b8b 100644
--- a/configs/Cubietruck_plus_defconfig
+++ b/configs/Cubietruck_plus_defconfig
@@ -16,6 +16,7 @@
 CONFIG_AXP_GPIO=y
 # CONFIG_SYS_MALLOC_CLEAR_ON_INIT is not set
 CONFIG_CONSOLE_MUX=y
+CONFIG_SPL_STACK=0x8000
 CONFIG_SYS_PBSIZE=1024
 CONFIG_SPL_SYS_I2C_LEGACY=y
 CONFIG_SYS_I2C_MVTWSI=y