kernel-arch: Fix buildpaths leaking into external module compiles

Building external kernel modules like lttng-modules was showing build paths
inside the debug symbols for the modules and breaking build reproducibility.

Fix this by adding in the mapping needed to map the kernel build directory
to something more approriate on target.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit b56dc900)
Signed-off-by: Steve Sakoman <steve@sakoman.com>