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/LicheePi_Zero_defconfig b/configs/LicheePi_Zero_defconfig
index 5bc36f2..2e0b0b7 100644
--- a/configs/LicheePi_Zero_defconfig
+++ b/configs/LicheePi_Zero_defconfig
@@ -5,5 +5,6 @@
 CONFIG_MACH_SUN8I_V3S=y
 CONFIG_DRAM_CLK=360
 # CONFIG_HAS_ARMV7_SECURE_BASE is not set
+CONFIG_SPL_STACK=0x8000
 CONFIG_SYS_PBSIZE=1024
 # CONFIG_NETDEVICES is not set