default or selected kernelversion does not have config value CONFIG_USB_SERIAL_FLASHLOADER.
Result is shown for kernelversion 5.10.117

Infineon Modem Flashloader USB interface driver

modulename: flashloader.ko

configname: CONFIG_USB_SERIAL_FLASHLOADER

Linux Kernel Configuration
└─>Device Drivers
└─>USB support
└─>USB Serial Converter support
└─>Infineon Modem Flashloader USB interface driver
In linux kernel since version 3.11 (release Date: 2013-09-02)  
Say Y here if you want to download Infineon Modem
via USB Flashloader serial driver.

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

source code: