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/apalis-tk1_defconfig b/configs/apalis-tk1_defconfig
index 548ff2e..3c00469 100644
--- a/configs/apalis-tk1_defconfig
+++ b/configs/apalis-tk1_defconfig
@@ -23,6 +23,8 @@
 CONFIG_ARCH_MISC_INIT=y
 CONFIG_SPL_FOOTPRINT_LIMIT=y
 CONFIG_SPL_MAX_FOOTPRINT=0x8000
+# CONFIG_SPL_SHARES_INIT_SP_ADDR is not set
+CONFIG_SPL_STACK=0x800ffffc
 CONFIG_SYS_PROMPT="Apalis TK1 # "
 CONFIG_SYS_MAXARGS=32
 CONFIG_SYS_CBSIZE=1024