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_one_plus_defconfig b/configs/orangepi_one_plus_defconfig
index e13a320..63d3add 100644
--- a/configs/orangepi_one_plus_defconfig
+++ b/configs/orangepi_one_plus_defconfig
@@ -7,6 +7,7 @@
 CONFIG_MMC0_CD_PIN="PF6"
 # CONFIG_PSCI_RESET is not set
 # CONFIG_SYS_MALLOC_CLEAR_ON_INIT is not set
+CONFIG_SPL_STACK=0x118000
 CONFIG_SYS_PBSIZE=1024
 CONFIG_USB_EHCI_HCD=y
 CONFIG_USB_OHCI_HCD=y