default or selected kernelversion does not have config value CONFIG_IEEE802154_FAKEHARD.
Result is shown for kernelversion 6.5.1

Fake LR-WPAN driver with several interconnected devices

modulename: fakehard.ko

configname: CONFIG_IEEE802154_FAKEHARD

Linux Kernel Configuration
└─>Device Drivers
└─>Network device support
└─>IEEE 802.15.4 drivers
└─>Fake LR-WPAN driver with several interconnected devices
In linux kernel since version 2.6.31 (release Date: 2009-09-09)  
Say Y here to enable the fake driver that serves as an example
of HardMAC device driver.

This driver can also be built as a module. To do so say M here.
The module will be called 'fakehard'.

source code: