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: 9dec5a0e ("nokia_rx51: disable obsolete VIDEO config")
Signed-off-by: NAnatolij Gustschin <agust@denx.de>
Reviewed-by: NPali Rohár <pali@kernel.org>

configs/nokia_rx51_defconfig

@@ -10,6 +10,7 @@ CONFIG_AUTOBOOT_KEYED=y
 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