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_nano_defconfig b/configs/licheepi_nano_defconfig
index 8502cf3..0252763 100644
--- a/configs/licheepi_nano_defconfig
+++ b/configs/licheepi_nano_defconfig
@@ -10,5 +10,6 @@
 CONFIG_DRAM_ZQ=0
 # CONFIG_VIDEO_SUNXI is not set
 CONFIG_SPL_SPI_SUNXI=y
+CONFIG_SPL_STACK=0x8000
 CONFIG_SYS_PBSIZE=1024
 # CONFIG_SYSRESET is not set