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/chromebit_mickey_defconfig b/configs/chromebit_mickey_defconfig
index 0225e10..862b37d 100644
--- a/configs/chromebit_mickey_defconfig
+++ b/configs/chromebit_mickey_defconfig
@@ -28,6 +28,8 @@
 CONFIG_SPL_PAD_TO=0x7f8000
 CONFIG_SPL_NO_BSS_LIMIT=y
 # CONFIG_SPL_RAW_IMAGE_SUPPORT is not set
+# CONFIG_SPL_SHARES_INIT_SP_ADDR is not set
+CONFIG_SPL_STACK=0xff718000
 CONFIG_SPL_STACK_R=y
 CONFIG_SPL_STACK_R_MALLOC_SIMPLE_LEN=0x2000
 # CONFIG_SYS_MMCSD_RAW_MODE_U_BOOT_USE_SECTOR is not set