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/pinecube_defconfig b/configs/pinecube_defconfig
index 1259b85..531cf0f 100644
--- a/configs/pinecube_defconfig
+++ b/configs/pinecube_defconfig
@@ -8,6 +8,7 @@
 CONFIG_DRAM_ODT_EN=y
 CONFIG_I2C0_ENABLE=y
 # CONFIG_HAS_ARMV7_SECURE_BASE is not set
+CONFIG_SPL_STACK=0x8000
 CONFIG_SPL_I2C=y
 CONFIG_SYS_PBSIZE=1024
 CONFIG_SPL_SYS_I2C_LEGACY=y