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/Bananapro_defconfig b/configs/Bananapro_defconfig
index 9214a9b..19b6446 100644
--- a/configs/Bananapro_defconfig
+++ b/configs/Bananapro_defconfig
@@ -11,6 +11,7 @@
 CONFIG_GMAC_TX_DELAY=3
 CONFIG_AHCI=y
 # CONFIG_SYS_MALLOC_CLEAR_ON_INIT is not set
+CONFIG_SPL_STACK=0x8000
 CONFIG_SPL_I2C=y
 CONFIG_SYS_PBSIZE=1024
 CONFIG_NETCONSOLE=y