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_mini_defconfig b/configs/Orangepi_mini_defconfig
index 974826b..494edf0 100644
--- a/configs/Orangepi_mini_defconfig
+++ b/configs/Orangepi_mini_defconfig
@@ -14,6 +14,7 @@
 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_SCSI_AHCI=y