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/chromebook_kevin_defconfig b/configs/chromebook_kevin_defconfig
index 5d95c9e..7f773fc 100644
--- a/configs/chromebook_kevin_defconfig
+++ b/configs/chromebook_kevin_defconfig
@@ -34,6 +34,8 @@
 CONFIG_SPL_BSS_MAX_SIZE=0x10000
 CONFIG_HANDOFF=y
 # CONFIG_SPL_RAW_IMAGE_SUPPORT is not set
+# CONFIG_SPL_SHARES_INIT_SP_ADDR is not set
+CONFIG_SPL_STACK=0xff8effff
 CONFIG_SPL_STACK_R=y
 CONFIG_SPL_STACK_R_MALLOC_SIMPLE_LEN=0x4000
 CONFIG_SPL_SPI_LOAD=y