This enables the firmware loading facility in the kernel. The kernel
will first look for built-in firmware, if it has any. Next, it will
look for the requested firmware in a series of filesystem paths:

o firmware_class path module parameter or kernel boot param
o /lib/firmware/updates/UTS_RELEASE
o /lib/firmware/updates
o /lib/firmware/UTS_RELEASE
o /lib/firmware

Enabling this feature only increases your kernel image by about
828 bytes, enable this option unless you are certain you don't
need firmware.

You typically want this built-in (=y) but you can also enable this
as a module, in which case the firmware_class module will be built.
You also want to be sure to enable this built-in if you are going to
enable built-in firmware (CONFIG_EXTRA_FIRMWARE).

source code:

drivers/base//firmware_loader/