kbuild: specify absolute paths for cscope

Cscope doesn't hadle relative paths when cscope.out is not in $PWD. Use
absolute paths when generating cscope.files, which seems to be the
recommended way to generate cscope.out, anyway (at least according to
cscope.sf.net). The speed and size differences are minimal, the only
drawback is that the database needs to be regenerated if the source
directory is moved.

[mmarek: fixed for O= builds, modified changelog]

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Michal Marek <mmarek@suse.cz>