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/kontron_sl28_defconfig b/configs/kontron_sl28_defconfig
index 9f4be1a..345db03 100644
--- a/configs/kontron_sl28_defconfig
+++ b/configs/kontron_sl28_defconfig
@@ -40,6 +40,8 @@
 CONFIG_SPL_MAX_SIZE=0x20000
 CONFIG_SPL_BSS_MAX_SIZE=0x100000
 CONFIG_SPL_BOARD_INIT=y
+# CONFIG_SPL_SHARES_INIT_SP_ADDR is not set
+CONFIG_SPL_STACK=0x18009ff0
 CONFIG_SPL_MPC8XXX_INIT_DDR=y
 CONFIG_SPL_SPI_LOAD=y
 CONFIG_SYS_SPI_U_BOOT_OFFS=0x230000