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/nanopi_neo_defconfig b/configs/nanopi_neo_defconfig
index e46e5b8..f837753 100644
--- a/configs/nanopi_neo_defconfig
+++ b/configs/nanopi_neo_defconfig
@@ -7,6 +7,7 @@
 # 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