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/Wobo_i5_defconfig b/configs/Wobo_i5_defconfig
index 95279c3..ab919c0 100644
--- a/configs/Wobo_i5_defconfig
+++ b/configs/Wobo_i5_defconfig
@@ -7,6 +7,7 @@
 CONFIG_MMC0_CD_PIN="PB3"
 CONFIG_USB1_VBUS_PIN="PG12"
 # CONFIG_SYS_MALLOC_CLEAR_ON_INIT is not set
+CONFIG_SPL_STACK=0x8000
 CONFIG_SPL_I2C=y
 CONFIG_SYS_PBSIZE=1024
 CONFIG_SYS_I2C_MVTWSI=y