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_gs1_defconfig b/configs/beelink_gs1_defconfig
index 339e6c9..6453a72 100644
--- a/configs/beelink_gs1_defconfig
+++ b/configs/beelink_gs1_defconfig
@@ -8,6 +8,7 @@
 CONFIG_MMC_SUNXI_SLOT_EXTRA=2
 # CONFIG_PSCI_RESET is not set
 # CONFIG_SYS_MALLOC_CLEAR_ON_INIT is not set
+CONFIG_SPL_STACK=0x118000
 CONFIG_SYS_PBSIZE=1024
 CONFIG_LED=y
 CONFIG_LED_GPIO=y