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/gardena-smart-gateway-mt7688_defconfig b/configs/gardena-smart-gateway-mt7688_defconfig
index 41496f7..61060909 100644
--- a/configs/gardena-smart-gateway-mt7688_defconfig
+++ b/configs/gardena-smart-gateway-mt7688_defconfig
@@ -74,6 +74,7 @@
 CONFIG_MTD_UBI_BEB_LIMIT=22
 CONFIG_MT7628_ETH=y
 CONFIG_PHY=y
+CONFIG_SPECIFY_CONSOLE_INDEX=y
 CONFIG_SPI=y
 CONFIG_MT7621_SPI=y
 CONFIG_WDT=y