kbuild: Use relative path for $(objtree)

The main Makefile sets its working directory to the object tree and
never changes it again. Therefore, we can use '.' instead of the
absolute path. The only case where we need the absolute path is when
creating the 'build' symlink in /lib/modules.

Acked-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Michal Marek <mmarek@suse.cz>