htools: change absolute to relative symlinks

Currently we use absolute symlinks, but this doesn't work when we
install remotely (due to install first to local temp dir, then rsync
to remote machines). To fix, we change to manually-computed relative
paths, which is not best, but it works.

One possible alternative would be to use hard-links…

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>