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/pinebook_defconfig b/configs/pinebook_defconfig
index 1bdfcf2..bf070aa 100644
--- a/configs/pinebook_defconfig
+++ b/configs/pinebook_defconfig
@@ -8,6 +8,7 @@
 CONFIG_DRAM_ZQ=3881949
 CONFIG_MMC_SUNXI_SLOT_EXTRA=2
 CONFIG_R_I2C_ENABLE=y
+CONFIG_SPL_STACK=0x54000
 CONFIG_SYS_PBSIZE=1024
 CONFIG_SPL_SYS_I2C_LEGACY=y
 CONFIG_SYS_I2C_MVTWSI=y