sh: Use relative paths for mach/cpu symlinks.

When building the kernel without passing the O= command line parameter
there's no point to use absolute paths for them.

Usually relative paths are preferred because they survive directory
moves, work across networked file systems and chrooted environments.

Absolute paths are still used if an output directory is given.

Signed-off-by: Franck Bui-Huu <fbuihuu@gmail.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>