FreeBSD on macbook & backlight module

Hello,
 
The wiki page about FreeBSD on MacBook explains that it's possible to
build a module to control backlight. A link points a depot where to
download backlight.c and backlightvar.h but there's no README.

I've tried to create a directory /usr/src/sys/dev/backlight put files in
it, add a line "device backlight" in /usr/src/sys/i386/conf/GENERIC
but make buildkernel ends with:
config: Error: device "backlight" is unknown
config: 1 errors
*** Error code 1
 
Is there someone who knows where to put these files under /usr/src and
compile a new kernel ?
Thanks for any help.

Michel.