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/Yones_Toptech_BS1078_V2_defconfig b/configs/Yones_Toptech_BS1078_V2_defconfig
index a14881b..ef30aee 100644
--- a/configs/Yones_Toptech_BS1078_V2_defconfig
+++ b/configs/Yones_Toptech_BS1078_V2_defconfig
@@ -16,6 +16,7 @@
 CONFIG_VIDEO_LCD_BL_PWM="PH13"
 CONFIG_VIDEO_LCD_PANEL_LVDS=y
 # CONFIG_SYS_MALLOC_CLEAR_ON_INIT is not set
+CONFIG_SPL_STACK=0x8000
 CONFIG_SYS_PBSIZE=1024
 CONFIG_AXP_DLDO1_VOLT=3300
 CONFIG_USB_MUSB_HOST=y