dbus: fix rebuild for boards where /var/lib isn't a symlink

Based on patch by Sagaert Johan <sagaert.johan@skynet.be>

Dbus rebuilds fails if /var/lib/dbus is a symlink (which it will be),
and the existing workaround only worked if /var/lib is a symlink to
/tmp, which isn't always the case.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>