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/beelink_x2_defconfig b/configs/beelink_x2_defconfig
index 432079a..4065e64 100644
--- a/configs/beelink_x2_defconfig
+++ b/configs/beelink_x2_defconfig
@@ -6,6 +6,7 @@
 CONFIG_DRAM_CLK=567
 CONFIG_MMC_SUNXI_SLOT_EXTRA=2
 # CONFIG_SYS_MALLOC_CLEAR_ON_INIT is not set
+CONFIG_SPL_STACK=0x8000
 CONFIG_SYS_PBSIZE=1024
 CONFIG_SUN8I_EMAC=y
 CONFIG_USB_EHCI_HCD=y