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/zeropi_defconfig b/configs/zeropi_defconfig
index dd0dbc9..7d45440 100644
--- a/configs/zeropi_defconfig
+++ b/configs/zeropi_defconfig
@@ -8,6 +8,7 @@
 # CONFIG_VIDEO_DE2 is not set
 # CONFIG_SYS_MALLOC_CLEAR_ON_INIT is not set
 CONFIG_CONSOLE_MUX=y
+CONFIG_SPL_STACK=0x8000
 CONFIG_SYS_PBSIZE=1024
 CONFIG_SUN8I_EMAC=y
 CONFIG_USB_EHCI_HCD=y