configs: iot2050: Disable CONFIG_CONSOLE_MUX

We only have serial as console option, and leaving this on turns on
SYS_CONSOLE_IS_IN_ENV which is also not true for these devices, leaving
an ugly

In:    No input devices available!
Out:   No output devices available!
Err:   No error devices available!

behind.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>