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: Anatolij Gustschin <agust@denx.de>
Reviewed-by: Pali Rohár <pali@kernel.org>