systemd: Add 'no-dns-fallback' PACKAGECONFIG option

systemd: Add 'no-dns-fallback' PACKAGECONFIG option

systemd defines a default set of fallback DNS servers in
https://github.com/systemd/systemd/blob/v251/meson_options.txt#L328-L330

By adding a PACKAGECONFIG knob providing a convenient way to opt out,
and then adding that value to systemd's PACKAGECONFIG, the output from
runtime 'resolvectl status' command no longer contains the following
line:

  Fallback DNS Servers: 1.1.1.1#cloudflare-dns.com 8.8.8.8#dns.google 1.0.0.1#cloudflare-dns.com 8.8.4.4#dns.google 2606:4700:4700::1111#cloudflare-dns.com 2001:4860:4860::8888#dns.google 2606:4700:4700::1001#cloudflare-dns.com 2001:4860:4860::8844#dns.google

(From OE-Core rev: 2b300d6b9ec6288a99d9dacb24a86949caf99e55)

Signed-off-by: Niko Mauno <niko.mauno@vaisala.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Steve Sakoman <steve@sakoman.com>