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/Bananapi_m2m_defconfig b/configs/Bananapi_m2m_defconfig
index ba26aa1..6a07f26 100644
--- a/configs/Bananapi_m2m_defconfig
+++ b/configs/Bananapi_m2m_defconfig
@@ -10,6 +10,7 @@
 CONFIG_MMC_SUNXI_SLOT_EXTRA=2
 CONFIG_USB0_ID_DET="PH8"
 # CONFIG_SYS_MALLOC_CLEAR_ON_INIT is not set
+CONFIG_SPL_STACK=0x8000
 CONFIG_SYS_PBSIZE=1024
 CONFIG_FASTBOOT_CMD_OEM_FORMAT=y
 CONFIG_USB_EHCI_HCD=y