default or selected kernelversion does not have config value CONFIG_SERIAL_NETX.
Result is shown for kernelversion 6.9.9

NetX serial port support

modulename: netx-serial.ko

configname: CONFIG_SERIAL_NETX

Linux Kernel Configuration
└─>Device Drivers
└─>Character devices
└─>Serial drivers
└─>NetX serial port support
In linux kernel since version 2.6.20 (release Date: 2007-02-04)  
If you have a machine based on a Hilscher NetX SoC you
can enable its onboard serial port by enabling this option.

To compile this driver as a module, choose M here: the
module will be called netx-serial.

source code: