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/Cubieboard4_defconfig b/configs/Cubieboard4_defconfig
index 82b3ac1..928299e 100644
--- a/configs/Cubieboard4_defconfig
+++ b/configs/Cubieboard4_defconfig
@@ -12,6 +12,7 @@
 CONFIG_USB1_VBUS_PIN="PH14"
 CONFIG_USB3_VBUS_PIN="PH15"
 CONFIG_AXP_GPIO=y
+CONFIG_SPL_STACK=0x18000
 CONFIG_SYS_PBSIZE=1024
 CONFIG_SYS_I2C_SUN8I_RSB=y
 CONFIG_AXP809_POWER=y