kbuild: add a symlink to the source for separate objdirs

I have some scripts which need to map back to the source directory
from an objdir. This was so far done by parsing the Makefile,
but the Makefile format changes occasionally and breaks my scripts
then.

To make this more reliable add a "source" symlink back.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>