Convert CONFIG_CONS_INDEX to Kconfig

This converts the following to Kconfig:
   CONFIG_CONS_INDEX

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/mscc_luton_defconfig b/configs/mscc_luton_defconfig
index d563901..0846ee8 100644
--- a/configs/mscc_luton_defconfig
+++ b/configs/mscc_luton_defconfig
@@ -72,6 +72,7 @@
 CONFIG_MSCC_LUTON_SWITCH=y
 CONFIG_PINCTRL=y
 CONFIG_PINCONF=y
+CONFIG_SPECIFY_CONSOLE_INDEX=y
 CONFIG_DM_SERIAL=y
 CONFIG_DEBUG_UART_SHIFT=2
 CONFIG_SYS_NS16550=y