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/Lamobo_R1_defconfig b/configs/Lamobo_R1_defconfig
index 8b25863..d4692f8 100644
--- a/configs/Lamobo_R1_defconfig
+++ b/configs/Lamobo_R1_defconfig
@@ -10,6 +10,7 @@
 CONFIG_GMAC_TX_DELAY=4
 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