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/orangepi_zero_defconfig b/configs/orangepi_zero_defconfig
index e6af0fb..b5ff84a 100644
--- a/configs/orangepi_zero_defconfig
+++ b/configs/orangepi_zero_defconfig
@@ -8,6 +8,7 @@
 CONFIG_SPL_SPI_SUNXI=y
 # CONFIG_SYS_MALLOC_CLEAR_ON_INIT is not set
 CONFIG_CONSOLE_MUX=y
+CONFIG_SPL_STACK=0x8000
 CONFIG_SYS_PBSIZE=1024
 CONFIG_SPI_FLASH_WINBOND=y
 CONFIG_SUN8I_EMAC=y