Hello,

 From personally building the kernel a few months ago (reproducibly) one 
of the parts that was incorrect in the docs was that is says to use 
KCFLAGS when you should use KCPPFLAGS for the following. I had forgotten 
to report this until now and I'm not sure exactly what is expected to 
make a patch to the Kernel docs, so I am hoping someone else can do the 
probably one line fix (or maybe 2 if you count the other reference to 
KCFLAGS in the file).

When the kernel is built out-of-tree, debug information may include
absolute filenames for the source files.  This must be overridden by
including the ``-fdebug-prefix-map`` option in the `KCFLAGS`_ variable.

from `Documentation/kbuild/reproducible-builds.rst`.

Regards,
Alex Coffin