You have to put the firmware files into the firmware directory in the kernel source tree or set CONFIG_EXTRA_FIRMWARE_DIR.
Originally Posted by xiando
This option controls the directory in which the kernel build system
looks for the firmware files listed in the EXTRA_FIRMWARE option.
The default is the firmware/ directory in the kernel source tree,
but by changing this option you can point it elsewhere, such as the
/lib/firmware/ directory or another separate directory containing