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/edminiv2_defconfig b/configs/edminiv2_defconfig
index ace1f76..dd6118b 100644
--- a/configs/edminiv2_defconfig
+++ b/configs/edminiv2_defconfig
@@ -25,6 +25,8 @@
 CONFIG_SPL_MAX_SIZE=0xfff0
 CONFIG_SPL_BSS_MAX_SIZE=0x1ffff
 CONFIG_SPL_BOARD_INIT=y
+# CONFIG_SPL_SHARES_INIT_SP_ADDR is not set
+CONFIG_SPL_STACK=0x20000
 CONFIG_SPL_NOR_SUPPORT=y
 CONFIG_HUSH_PARSER=y
 # CONFIG_AUTO_COMPLETE is not set