layer.conf: Add os-release to SIGGEN_EXCLUDERECIPES_ABISAFE
This rebuilds whenever DISTRO_VERSION changes (i.e. any commit is made)
and systemd depends upon it so lots of things end up rebuilding which
isn't necessary,
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org> Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>