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/icnova-a20-swac_defconfig b/configs/icnova-a20-swac_defconfig
index 44080b9..afe5b28 100644
--- a/configs/icnova-a20-swac_defconfig
+++ b/configs/icnova-a20-swac_defconfig
@@ -19,6 +19,7 @@
 CONFIG_VIDEO_LCD_POWER="PH22"
 CONFIG_VIDEO_LCD_PANEL_LVDS=y
 # CONFIG_SYS_MALLOC_CLEAR_ON_INIT is not set
+CONFIG_SPL_STACK=0x8000
 CONFIG_SPL_I2C=y
 CONFIG_SYS_PBSIZE=1024
 CONFIG_CMD_UNZIP=y