kernel-devsrc: fix /usr/src/kernel symlink to also work for SDK
If kernel-devsrc is used in a SDK context, the symlink from usr/src/kernel
points to an absolute path '/lib/modules/${KERNEL_VERSION}/build',
which ends up pointing to the local machine's build directory.
To address this issue change the symlink to be realtive to
${D}/lib/modules/${KERNEL_VERSION}/build.
Signed-off-by: Henrik Nymann Jensen <hnje@triax.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
