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/inet_q972_defconfig b/configs/inet_q972_defconfig
index 73700b1..a4a828c 100644
--- a/configs/inet_q972_defconfig
+++ b/configs/inet_q972_defconfig
@@ -15,6 +15,7 @@
 CONFIG_VIDEO_LCD_BL_EN="PA25"
 CONFIG_VIDEO_LCD_BL_PWM="PH13"
 # CONFIG_SYS_MALLOC_CLEAR_ON_INIT is not set
+CONFIG_SPL_STACK=0x8000
 CONFIG_SYS_PBSIZE=1024
 CONFIG_AXP_DLDO1_VOLT=3300
 CONFIG_USB_EHCI_HCD=y