nokia_rx51: re-enable CONSOLE_MUX and SYS_CONSOLE_IS_IN_ENV

With disabled legacy VIDEO option CONSOLE_MUX is not auto-selected
any more, re-enable it.

Fixes: 9dec5a0ea130 ("nokia_rx51: disable obsolete VIDEO config")
Signed-off-by: Anatolij Gustschin <agust@denx.de>
Reviewed-by: Pali Rohár <pali@kernel.org>
diff --git a/configs/nokia_rx51_defconfig b/configs/nokia_rx51_defconfig
index b0a58e5..1d35352 100644
--- a/configs/nokia_rx51_defconfig
+++ b/configs/nokia_rx51_defconfig
@@ -10,6 +10,7 @@
 CONFIG_AUTOBOOT_MENU_SHOW=y
 CONFIG_USE_PREBOOT=y
 CONFIG_PREBOOT="run preboot"
+CONFIG_CONSOLE_MUX=y
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="Nokia RX-51 # "
 # CONFIG_CMD_BDI is not set