local.conf.sample.extended: Mention shadow-base for login_manager for systemd
While not strictly required, list shadow-base as the example to use for
VIRTUAL-RUNTIME_login_manager when configuring for systemd. This
provider matches what is found in for example Debian and is also one of
the non-intuitive VIRTUAL-RUNTIME alternatives, so it's good to have an
example in use somewhere.
Signed-off-by: Tom Rini <trini@konsulko.com> Signed-off-by: Ross Burton <ross.burton@intel.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
