systemd: set zstd as default PACKAGECONFIG

Switch the default PACKAGECONFIG for compression feature from xz
to zstd.  zstd is significantly faster than xz with only slightly
worse compression ratios.  It is therefore much better suited for
activities like systemd-journald.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>